Introduce a new markdown crate (#11556)

This pull request introduces a new `markdown` crate which is capable of
parsing and rendering a Markdown source. One of the key additions is
that it enables text selection within a `Markdown` view. Eventually,
this will replace `RichText` but for now the goal is to use it in the
assistant revamped assistant in the spirit of making progress.

<img width="711" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b56c777b-e57c-42f9-95c1-3ada22f63a69">

Note that this pull request doesn't yet use the new markdown renderer in
`assistant2`. This is because we need to modify the assistant before
slotting in the new renderer and I wanted to merge this independently of
those changes.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Alp <akeles@umd.edu>
Co-authored-by: Zachiah Sawyer <zachiah@proton.me>

crates/markdown/examples/markdown.rs

@@ -0,0 +1,181 @@
+use assets::Assets;
+use gpui::{prelude::*, App, Task, View, WindowOptions};
+use language::{language_settings::AllLanguageSettings, LanguageRegistry};
+use markdown::{Markdown, MarkdownStyle};
+use node_runtime::FakeNodeRuntime;
+use settings::SettingsStore;
+use std::sync::Arc;
+use theme::LoadThemes;
+use ui::prelude::*;
+use ui::{div, WindowContext};
+
+const MARKDOWN_EXAMPLE: &'static str = r#"
+# Markdown Example Document
+
+## Headings
+Headings are created by adding one or more `#` symbols before your heading text. The number of `#` you use will determine the size of the heading.
+
+## Emphasis
+Emphasis can be added with italics or bold. *This text will be italic*. _This will also be italic_
+
+## Lists
+
+### Unordered Lists
+Unordered lists use asterisks `*`, plus `+`, or minus `-` as list markers.
+
+* Item 1
+* Item 2
+  * Item 2a
+  * Item 2b
+
+### Ordered Lists
+Ordered lists use numbers followed by a period.
+
+1. Item 1
+2. Item 2
+3. Item 3
+   1. Item 3a
+   2. Item 3b
+
+## Links
+Links are created using the format [http://zed.dev](https://zed.dev).
+
+They can also be detected automatically, for example https://zed.dev/blog.
+
+## Images
+Images are like links, but with an exclamation mark `!` in front.
+
+```todo!
+![This is an image](/images/logo.png)
+```
+
+## Code
+Inline `code` can be wrapped with backticks `` ` ``.
+
+```markdown
+Inline `code` has `back-ticks around` it.
+```
+
+Code blocks can be created by indenting lines by four spaces or with triple backticks ```.
+
+```javascript
+function test() {
+  console.log("notice the blank line before this function?");
+}
+```
+
+## Blockquotes
+Blockquotes are created with `>`.
+
+> This is a blockquote.
+
+## Horizontal Rules
+Horizontal rules are created using three or more asterisks `***`, dashes `---`, or underscores `___`.
+
+## Line breaks
+This is a
+\
+line break!
+
+---
+
+Remember, markdown processors may have slight differences and extensions, so always refer to the specific documentation or guides relevant to your platform or editor for the best practices and additional features.
+"#;
+
+pub fn main() {
+    env_logger::init();
+    App::new().with_assets(Assets).run(|cx| {
+        let store = SettingsStore::test(cx);
+        cx.set_global(store);
+        language::init(cx);
+        SettingsStore::update(cx, |store, cx| {
+            store.update_user_settings::<AllLanguageSettings>(cx, |_| {});
+        });
+
+        let node_runtime = FakeNodeRuntime::new();
+        let language_registry = Arc::new(LanguageRegistry::new(
+            Task::ready(()),
+            cx.background_executor().clone(),
+        ));
+        languages::init(language_registry.clone(), node_runtime, cx);
+        theme::init(LoadThemes::JustBase, cx);
+        Assets.load_fonts(cx).unwrap();
+
+        cx.activate(true);
+        cx.open_window(WindowOptions::default(), |cx| {
+            cx.new_view(|cx| {
+                MarkdownExample::new(
+                    MARKDOWN_EXAMPLE.to_string(),
+                    MarkdownStyle {
+                        code_block: gpui::TextStyleRefinement {
+                            font_family: Some("Zed Mono".into()),
+                            color: Some(cx.theme().colors().editor_foreground),
+                            background_color: Some(cx.theme().colors().editor_background),
+                            ..Default::default()
+                        },
+                        inline_code: gpui::TextStyleRefinement {
+                            font_family: Some("Zed Mono".into()),
+                            // @nate: Could we add inline-code specific styles to the theme?
+                            color: Some(cx.theme().colors().editor_foreground),
+                            background_color: Some(cx.theme().colors().editor_background),
+                            ..Default::default()
+                        },
+                        rule_color: Color::Muted.color(cx),
+                        block_quote_border_color: Color::Muted.color(cx),
+                        block_quote: gpui::TextStyleRefinement {
+                            color: Some(Color::Muted.color(cx)),
+                            ..Default::default()
+                        },
+                        link: gpui::TextStyleRefinement {
+                            color: Some(Color::Accent.color(cx)),
+                            underline: Some(gpui::UnderlineStyle {
+                                thickness: px(1.),
+                                color: Some(Color::Accent.color(cx)),
+                                wavy: false,
+                            }),
+                            ..Default::default()
+                        },
+                        syntax: cx.theme().syntax().clone(),
+                        selection_background_color: {
+                            let mut selection = cx.theme().players().local().selection;
+                            selection.fade_out(0.7);
+                            selection
+                        },
+                    },
+                    language_registry,
+                    cx,
+                )
+            })
+        });
+    });
+}
+
+struct MarkdownExample {
+    markdown: View<Markdown>,
+}
+
+impl MarkdownExample {
+    pub fn new(
+        text: String,
+        style: MarkdownStyle,
+        language_registry: Arc<LanguageRegistry>,
+        cx: &mut WindowContext,
+    ) -> Self {
+        let markdown = cx.new_view(|cx| Markdown::new(text, style, language_registry, cx));
+        Self { markdown }
+    }
+}
+
+impl Render for MarkdownExample {
+    fn render(&mut self, _cx: &mut ViewContext<Self>) -> impl IntoElement {
+        div()
+            .id("markdown-example")
+            .debug_selector(|| "foo".into())
+            .relative()
+            .bg(gpui::white())
+            .size_full()
+            .p_4()
+            .overflow_y_scroll()
+            .child(self.markdown.clone())
+    }
+}