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/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