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/rich_text/Cargo.toml

@@ -22,7 +22,7 @@ futures.workspace = true
 gpui = { path = "../gpui" }
 language = { path = "../language" }
 lazy_static.workspace = true
-pulldown-cmark = { version = "0.9.2", default-features = false }
+pulldown-cmark.workspace = true
 smallvec.workspace = true
 smol.workspace = true
 sum_tree = { path = "../sum_tree" }

crates/rich_text/src/rich_text.rs

@@ -47,7 +47,7 @@ pub struct Mention {
 }
 
 impl RichText {
-    pub fn element(&self, id: ElementId, cx: &mut WindowContext) -> AnyElement {
+    pub fn element(&self, id: ElementId, cx: &WindowContext) -> AnyElement {
         let theme = cx.theme();
         let code_background = theme.colors().surface_background;
 
@@ -83,7 +83,12 @@ impl RichText {
         )
         .on_click(self.link_ranges.clone(), {
             let link_urls = self.link_urls.clone();
-            move |ix, cx| cx.open_url(&link_urls[ix])
+            move |ix, cx| {
+                let url = &link_urls[ix];
+                if url.starts_with("http") {
+                    cx.open_url(url);
+                }
+            }
         })
         .tooltip({
             let link_ranges = self.link_ranges.clone();
@@ -256,7 +261,7 @@ pub fn render_markdown_mut(
     }
 }
 
-pub fn render_markdown(
+pub fn render_rich_text(
     block: String,
     mentions: &[Mention],
     language_registry: &Arc<LanguageRegistry>,