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/tab_map.rs

@@ -508,7 +508,7 @@ impl<'a> Iterator for TabChunks<'a> {
                         self.chunk.text = suffix;
                         return Some(Chunk {
                             text: prefix,
-                            ..self.chunk
+                            ..self.chunk.clone()
                         });
                     } else {
                         self.chunk.text = &self.chunk.text[1..];
@@ -529,7 +529,7 @@ impl<'a> Iterator for TabChunks<'a> {
                         return Some(Chunk {
                             text: &SPACES[..len as usize],
                             is_tab: true,
-                            ..self.chunk
+                            ..self.chunk.clone()
                         });
                     }
                 }