Switch from Arc/RwLock to Rc/RefCell for CodeContextMenu (#22035)

`CodeContextMenu` is always accessed on one thread, so only `Rc`s and
`Rc<RefCell<_>>` are needed. There should be tiny performance benefits
from this. The main benefit of this is that when seeing code accessing a
`RwLock` it would be reasonable to wonder whether it will block. The
only potential downside is the potential for panics due to overlapping
borrows of the RefCells. I think this is an acceptable risk because most
errors of this nature will be local or will be caught by clippy via the
check for holding a RefCell reference over an `await`.

Release Notes:

- N/A

crates/collab_ui/Cargo.toml

@@ -44,7 +44,6 @@ gpui.workspace = true
 language.workspace = true
 menu.workspace = true
 notifications.workspace = true
-parking_lot.workspace = true
 picker.workspace = true
 project.workspace = true
 release_channel.workspace = true

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -12,10 +12,15 @@ use language::{
     language_settings::SoftWrap, Anchor, Buffer, BufferSnapshot, CodeLabel, LanguageRegistry,
     LanguageServerId, ToOffset,
 };
-use parking_lot::RwLock;
 use project::{search::SearchQuery, Completion};
 use settings::Settings;
-use std::{ops::Range, sync::Arc, sync::LazyLock, time::Duration};
+use std::{
+    cell::RefCell,
+    ops::Range,
+    rc::Rc,
+    sync::{Arc, LazyLock},
+    time::Duration,
+};
 use theme::ThemeSettings;
 use ui::{prelude::*, TextSize};
 
@@ -68,7 +73,7 @@ impl CompletionProvider for MessageEditorCompletionProvider {
         &self,
         _buffer: Model<Buffer>,
         _completion_indices: Vec<usize>,
-        _completions: Arc<RwLock<Box<[Completion]>>>,
+        _completions: Rc<RefCell<Box<[Completion]>>>,
         _cx: &mut ViewContext<Editor>,
     ) -> Task<anyhow::Result<bool>> {
         Task::ready(Ok(false))