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/editor/src/display_map/block_map.rs

@@ -875,7 +875,7 @@ impl<'a> Iterator for BlockChunks<'a> {
 
         Some(Chunk {
             text: prefix,
-            ..self.input_chunk
+            ..self.input_chunk.clone()
         })
     }
 }