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/assistant2/src/assistant2.rs

@@ -869,7 +869,6 @@ impl AssistantChat {
                             crate::ui::ChatMessage::new(
                                 *id,
                                 UserOrAssistant::User(self.user_store.read(cx).current_user()),
-                                // todo!(): clean up the vec usage
                                 vec![
                                     body.clone().into_any_element(),
                                     h_flex()