Introduce custom fold placeholders (#12214)

This pull request replaces the static `⋯` character we used to insert
when folding a range with a custom render function that return an
`AnyElement`. We plan to use this in the assistant, but for now this
should be behavior-preserving.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>

crates/gpui/src/elements/text.rs

@@ -143,7 +143,7 @@ impl StyledText {
         }
     }
 
-    /// todo!()
+    /// Get the layout for this element. This can be used to map indices to pixels and vice versa.
     pub fn layout(&self) -> &TextLayout {
         &self.layout
     }
@@ -232,7 +232,7 @@ impl IntoElement for StyledText {
     }
 }
 
-/// todo!()
+/// The Layout for TextElement. This can be used to map indices to pixels and vice versa.
 #[derive(Default, Clone)]
 pub struct TextLayout(Arc<Mutex<Option<TextLayoutInner>>>);
 
@@ -358,7 +358,7 @@ impl TextLayout {
         }
     }
 
-    /// todo!()
+    /// Get the byte index into the input of the pixel position.
     pub fn index_for_position(&self, mut position: Point<Pixels>) -> Result<usize, usize> {
         let element_state = self.lock();
         let element_state = element_state
@@ -392,7 +392,7 @@ impl TextLayout {
         Err(line_start_ix.saturating_sub(1))
     }
 
-    /// todo!()
+    /// Get the pixel position for the given byte index.
     pub fn position_for_index(&self, index: usize) -> Option<Point<Pixels>> {
         let element_state = self.lock();
         let element_state = element_state
@@ -423,17 +423,17 @@ impl TextLayout {
         None
     }
 
-    /// todo!()
+    /// The bounds of this layout.
     pub fn bounds(&self) -> Bounds<Pixels> {
         self.0.lock().as_ref().unwrap().bounds.unwrap()
     }
 
-    /// todo!()
+    /// The line height for this layout.
     pub fn line_height(&self) -> Pixels {
         self.0.lock().as_ref().unwrap().line_height
     }
 
-    /// todo!()
+    /// The text for this layout.
     pub fn text(&self) -> String {
         self.0
             .lock()

crates/gpui/src/text_system/line_layout.rs

@@ -302,7 +302,7 @@ impl WrappedLineLayout {
         }
     }
 
-    /// todo!()
+    /// Returns the pixel position for the given byte index.
     pub fn position_for_index(&self, index: usize, line_height: Pixels) -> Option<Point<Pixels>> {
         let mut line_start_ix = 0;
         let mut line_end_indices = self