Indent guides (#11503)

Builds on top of existing work from #2249, but here's a showcase:


https://github.com/zed-industries/zed/assets/53836821/4b346965-6654-496c-b379-75425d9b493f

TODO:
- [x] handle line wrapping
- [x] implement handling in multibuffer (crashes currently)
- [x] add configuration option
- [x] new theme properties? What colors to use?
- [x] Possibly support indents with different colors or background
colors
- [x] investigate edge cases (e.g. indent guides and folds continue on
empty lines even if the next indent is different)
- [x] add more tests (also test `find_active_indent_index`)
- [x] docs (will do in a follow up PR)
- [x] benchmark performance impact

Release Notes:

- Added indent guides
([#5373](https://github.com/zed-industries/zed/issues/5373))

---------

Co-authored-by: Nate Butler <1714999+iamnbutler@users.noreply.github.com>
Co-authored-by: Remco <djsmits12@gmail.com>

crates/theme/src/schema.rs

@@ -75,6 +75,9 @@ pub struct ThemeStyleContent {
     #[serde(default, rename = "background.appearance")]
     pub window_background_appearance: Option<WindowBackgroundContent>,
 
+    #[serde(default)]
+    pub accents: Vec<AccentContent>,
+
     #[serde(flatten, default)]
     pub colors: ThemeColorsContent,
 
@@ -381,6 +384,12 @@ pub struct ThemeColorsContent {
     #[serde(rename = "editor.active_wrap_guide")]
     pub editor_active_wrap_guide: Option<String>,
 
+    #[serde(rename = "editor.indent_guide")]
+    pub editor_indent_guide: Option<String>,
+
+    #[serde(rename = "editor.indent_guide_active")]
+    pub editor_indent_guide_active: Option<String>,
+
     /// Read-access of a symbol, like reading a variable.
     ///
     /// A document highlight is a range inside a text document which deserves
@@ -747,6 +756,14 @@ impl ThemeColorsContent {
                 .editor_active_wrap_guide
                 .as_ref()
                 .and_then(|color| try_parse_color(color).ok()),
+            editor_indent_guide: self
+                .editor_indent_guide
+                .as_ref()
+                .and_then(|color| try_parse_color(color).ok()),
+            editor_indent_guide_active: self
+                .editor_indent_guide_active
+                .as_ref()
+                .and_then(|color| try_parse_color(color).ok()),
             editor_document_highlight_read_background: self
                 .editor_document_highlight_read_background
                 .as_ref()
@@ -1196,6 +1213,9 @@ impl StatusColorsContent {
     }
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
+pub struct AccentContent(pub Option<String>);
+
 #[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
 pub struct PlayerColorContent {
     pub cursor: Option<String>,