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/assistant/src/assistant.rs

@@ -189,7 +189,7 @@ pub struct LanguageModelChoiceDelta {
 struct MessageMetadata {
     role: Role,
     status: MessageStatus,
-    // todo!("delete this")
+    // TODO: Delete this
     #[serde(skip)]
     ambient_context: AmbientContextSnapshot,
 }

crates/assistant/src/assistant_panel.rs

@@ -19,6 +19,7 @@ use crate::{
 use anyhow::{anyhow, Result};
 use client::telemetry::Telemetry;
 use collections::{hash_map, HashMap, HashSet, VecDeque};
+use editor::FoldPlaceholder;
 use editor::{
     actions::{FoldAt, MoveDown, MoveUp},
     display_map::{
@@ -34,7 +35,7 @@ use fs::Fs;
 use futures::StreamExt;
 use gpui::{
     canvas, div, point, relative, rems, uniform_list, Action, AnyElement, AnyView, AppContext,
-    AsyncAppContext, AsyncWindowContext, AvailableSpace, ClipboardItem, Context, Entity,
+    AsyncAppContext, AsyncWindowContext, AvailableSpace, ClipboardItem, Context, Empty, Entity,
     EventEmitter, FocusHandle, FocusableView, FontStyle, FontWeight, HighlightStyle,
     InteractiveElement, IntoElement, Model, ModelContext, ParentElement, Pixels, Render,
     SharedString, StatefulInteractiveElement, Styled, Subscription, Task, TextStyle,
@@ -2943,6 +2944,10 @@ impl ConversationEditor {
                         .insert_flaps(
                             [Flap::new(
                                 start..end,
+                                FoldPlaceholder {
+                                    render: Arc::new(|_, _, _| Empty.into_any()),
+                                    constrain_width: false,
+                                },
                                 render_slash_command_output_toggle,
                                 render_slash_command_output_trailer,
                             )],