Introduce an outline panel (#12637)

Adds a new panel: `OutlinePanel` which looks very close to project
panel:

<img width="256" alt="Screenshot 2024-06-10 at 23 19 05"
src="https://github.com/zed-industries/zed/assets/2690773/c66e6e78-44ec-4de8-8d60-43238bb09ae9">

has similar settings and keymap (actions work in the `OutlinePanel`
context and are under `outline_panel::` namespace), with two notable
differences:
* no "edit" actions such as cut/copy/paste/delete/etc.
* directory auto folding is enabled by default

Empty view: 
<img width="841" alt="Screenshot 2024-06-10 at 23 19 11"
src="https://github.com/zed-industries/zed/assets/2690773/dc8bf37c-5a70-4fd5-9b57-76271eb7a40c">


When editor gets active, the panel displays all related files in a tree
(similar to what the project panel does) and all related excerpts'
outlines under each file.
Same as in the project panel, directories can be expanded or collapsed,
unfolded or folded; clicking file entries or outlines scrolls the buffer
to the corresponding excerpt; changing editor's selection reveals the
corresponding outline in the panel.

The panel is applicable to any singleton buffer:
<img width="1215" alt="Screenshot 2024-06-10 at 23 19 35"
src="https://github.com/zed-industries/zed/assets/2690773/a087631f-5c2d-4d4d-ae25-30ab9731d528">

<img width="1728" alt="image"
src="https://github.com/zed-industries/zed/assets/2690773/e4f8082c-d12d-4473-8500-e8fd1051285b">

or any multi buffer:

(search multi buffer)

<img width="1728" alt="Screenshot 2024-06-10 at 23 19 41"
src="https://github.com/zed-industries/zed/assets/2690773/60f768a3-6716-4520-9b13-42da8fd15f50">

(diagnostics multi buffer)
<img width="1728" alt="image"
src="https://github.com/zed-industries/zed/assets/2690773/64e285bd-9530-4bf2-8f1f-10ee5596067c">

Release Notes:
- Added an outline panel to show a "map" of the active editor

crates/language/src/buffer.rs

@@ -2738,12 +2738,13 @@ impl BufferSnapshot {
         Some(items)
     }
 
-    fn outline_items_containing(
+    pub fn outline_items_containing<T: ToOffset>(
         &self,
-        range: Range<usize>,
+        range: Range<T>,
         include_extra_context: bool,
         theme: Option<&SyntaxTheme>,
     ) -> Option<Vec<OutlineItem<Anchor>>> {
+        let range = range.to_offset(self);
         let mut matches = self.syntax.matches(range.clone(), &self.text, |grammar| {
             grammar.outline_config.as_ref().map(|c| &c.query)
         });

crates/language/src/language.rs

@@ -70,7 +70,7 @@ pub use language_registry::{
     PendingLanguageServer, QUERY_FILENAME_PREFIXES,
 };
 pub use lsp::LanguageServerId;
-pub use outline::{Outline, OutlineItem};
+pub use outline::{render_item, Outline, OutlineItem};
 pub use syntax_map::{OwnedSyntaxLayer, SyntaxLayer};
 pub use text::{AnchorRangeExt, LineEnding};
 pub use tree_sitter::{Node, Parser, Tree, TreeCursor};

crates/language/src/outline.rs

@@ -1,6 +1,11 @@
 use fuzzy::{StringMatch, StringMatchCandidate};
-use gpui::{BackgroundExecutor, HighlightStyle};
+use gpui::{
+    relative, AppContext, BackgroundExecutor, FontStyle, FontWeight, HighlightStyle, StyledText,
+    TextStyle, WhiteSpace,
+};
+use settings::Settings;
 use std::ops::Range;
+use theme::{ActiveTheme, ThemeSettings};
 
 /// An outline of all the symbols contained in a buffer.
 #[derive(Debug)]
@@ -11,7 +16,7 @@ pub struct Outline<T> {
     path_candidate_prefixes: Vec<usize>,
 }
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct OutlineItem<T> {
     pub depth: usize,
     pub range: Range<T>,
@@ -138,3 +143,34 @@ impl<T> Outline<T> {
         tree_matches
     }
 }
+
+pub fn render_item<T>(
+    outline_item: &OutlineItem<T>,
+    custom_highlights: impl IntoIterator<Item = (Range<usize>, HighlightStyle)>,
+    cx: &AppContext,
+) -> StyledText {
+    let settings = ThemeSettings::get_global(cx);
+
+    // TODO: We probably shouldn't need to build a whole new text style here
+    // but I'm not sure how to get the current one and modify it.
+    // Before this change TextStyle::default() was used here, which was giving us the wrong font and text color.
+    let text_style = TextStyle {
+        color: cx.theme().colors().text,
+        font_family: settings.buffer_font.family.clone(),
+        font_features: settings.buffer_font.features.clone(),
+        font_size: settings.buffer_font_size(cx).into(),
+        font_weight: FontWeight::NORMAL,
+        font_style: FontStyle::Normal,
+        line_height: relative(1.),
+        background_color: None,
+        underline: None,
+        strikethrough: None,
+        white_space: WhiteSpace::Normal,
+    };
+    let highlights = gpui::combine_highlights(
+        custom_highlights,
+        outline_item.highlight_ranges.iter().cloned(),
+    );
+
+    StyledText::new(outline_item.text.clone()).with_highlights(&text_style, highlights)
+}