Add initial markdown preview to Zed (#6958)

Adds a "markdown: open preview" action to open a markdown preview.

https://github.com/zed-industries/zed/assets/18583882/6fd7f009-53f7-4f98-84ea-7dd3f0dd11bf


This PR extends the work done in `crates/rich_text` to render markdown
to also support:

- Variable heading sizes
- Markdown tables
- Code blocks
- Block quotes

## Release Notes

- Added `Markdown: Open preview` action to partially close
([#6789](https://github.com/zed-industries/zed/issues/6789)).

## Known issues that will not be included in this PR

- Images.
- Nested block quotes.
- Footnote Reference.
- Headers highlighting.
- Inline code highlighting (this will need to be implemented in
`rich_text`)
- Checkboxes (`- [ ]` and `- [x]`)
- Syntax highlighting in code blocks.
- Markdown table text alignment.
- Inner markdown URL clicks

crates/markdown_preview/src/markdown_preview_view.rs

@@ -0,0 +1,134 @@
+use editor::{Editor, EditorEvent};
+use gpui::{
+    canvas, AnyElement, AppContext, AvailableSpace, EventEmitter, FocusHandle, FocusableView,
+    InteractiveElement, IntoElement, ParentElement, Render, Styled, View, ViewContext,
+};
+use language::LanguageRegistry;
+use std::sync::Arc;
+use ui::prelude::*;
+use workspace::item::Item;
+use workspace::Workspace;
+
+use crate::{markdown_renderer::render_markdown, OpenPreview};
+
+pub struct MarkdownPreviewView {
+    focus_handle: FocusHandle,
+    languages: Arc<LanguageRegistry>,
+    contents: String,
+}
+
+impl MarkdownPreviewView {
+    pub fn register(workspace: &mut Workspace, _cx: &mut ViewContext<Workspace>) {
+        let languages = workspace.app_state().languages.clone();
+
+        workspace.register_action(move |workspace, _: &OpenPreview, cx| {
+            if workspace.has_active_modal(cx) {
+                cx.propagate();
+                return;
+            }
+            let languages = languages.clone();
+            if let Some(editor) = workspace.active_item_as::<Editor>(cx) {
+                let view: View<MarkdownPreviewView> =
+                    cx.new_view(|cx| MarkdownPreviewView::new(editor, languages, cx));
+                workspace.split_item(workspace::SplitDirection::Right, Box::new(view.clone()), cx);
+                cx.notify();
+            }
+        });
+    }
+
+    pub fn new(
+        active_editor: View<Editor>,
+        languages: Arc<LanguageRegistry>,
+        cx: &mut ViewContext<Self>,
+    ) -> Self {
+        let focus_handle = cx.focus_handle();
+
+        cx.subscribe(&active_editor, |this, editor, event: &EditorEvent, cx| {
+            if *event == EditorEvent::Edited {
+                let editor = editor.read(cx);
+                let contents = editor.buffer().read(cx).snapshot(cx).text();
+                this.contents = contents;
+                cx.notify();
+            }
+        })
+        .detach();
+
+        let editor = active_editor.read(cx);
+        let contents = editor.buffer().read(cx).snapshot(cx).text();
+
+        Self {
+            focus_handle,
+            languages,
+            contents,
+        }
+    }
+}
+
+impl FocusableView for MarkdownPreviewView {
+    fn focus_handle(&self, _: &AppContext) -> gpui::FocusHandle {
+        self.focus_handle.clone()
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum PreviewEvent {}
+
+impl EventEmitter<PreviewEvent> for MarkdownPreviewView {}
+
+impl Item for MarkdownPreviewView {
+    type Event = PreviewEvent;
+
+    fn tab_content(
+        &self,
+        _detail: Option<usize>,
+        selected: bool,
+        _cx: &WindowContext,
+    ) -> AnyElement {
+        h_flex()
+            .gap_2()
+            .child(Icon::new(IconName::FileDoc).color(if selected {
+                Color::Default
+            } else {
+                Color::Muted
+            }))
+            .child(Label::new("Markdown preview").color(if selected {
+                Color::Default
+            } else {
+                Color::Muted
+            }))
+            .into_any()
+    }
+
+    fn telemetry_event_text(&self) -> Option<&'static str> {
+        Some("markdown preview")
+    }
+
+    fn to_item_events(_event: &Self::Event, _f: impl FnMut(workspace::item::ItemEvent)) {}
+}
+
+impl Render for MarkdownPreviewView {
+    fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
+        let rendered_markdown = v_flex()
+            .items_start()
+            .justify_start()
+            .key_context("MarkdownPreview")
+            .track_focus(&self.focus_handle)
+            .id("MarkdownPreview")
+            .overflow_scroll()
+            .size_full()
+            .bg(cx.theme().colors().editor_background)
+            .p_4()
+            .children(render_markdown(&self.contents, &self.languages, cx));
+
+        div().flex_1().child(
+            canvas(move |bounds, cx| {
+                rendered_markdown.into_any().draw(
+                    bounds.origin,
+                    bounds.size.map(AvailableSpace::Definite),
+                    cx,
+                )
+            })
+            .size_full(),
+        )
+    }
+}