project panel: Add indent guides (#18260)

See #12673



https://github.com/user-attachments/assets/94079afc-a851-4206-9c9b-4fad3542334e



TODO:
- [x] Make active indent guides work for autofolded directories
- [x] Figure out which theme colors to use
- [x] Fix horizontal scrolling
- [x] Make indent guides easier to click
- [x] Fix selected background flashing when hovering over entry/indent
guide
- [x] Docs

Release Notes:

- Added indent guides to the project panel

crates/theme/src/schema.rs

@@ -322,6 +322,15 @@ pub struct ThemeColorsContent {
     #[serde(rename = "panel.focused_border")]
     pub panel_focused_border: Option<String>,
 
+    #[serde(rename = "panel.indent_guide")]
+    pub panel_indent_guide: Option<String>,
+
+    #[serde(rename = "panel.indent_guide_hover")]
+    pub panel_indent_guide_hover: Option<String>,
+
+    #[serde(rename = "panel.indent_guide_active")]
+    pub panel_indent_guide_active: Option<String>,
+
     #[serde(rename = "pane.focused_border")]
     pub pane_focused_border: Option<String>,
 
@@ -710,6 +719,18 @@ impl ThemeColorsContent {
                 .panel_focused_border
                 .as_ref()
                 .and_then(|color| try_parse_color(color).ok()),
+            panel_indent_guide: self
+                .panel_indent_guide
+                .as_ref()
+                .and_then(|color| try_parse_color(color).ok()),
+            panel_indent_guide_hover: self
+                .panel_indent_guide_hover
+                .as_ref()
+                .and_then(|color| try_parse_color(color).ok()),
+            panel_indent_guide_active: self
+                .panel_indent_guide_active
+                .as_ref()
+                .and_then(|color| try_parse_color(color).ok()),
             pane_focused_border: self
                 .pane_focused_border
                 .as_ref()