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/assistant/src/inline_assistant.rs

@@ -47,6 +47,7 @@ use std::{
     iter, mem,
     ops::{Range, RangeInclusive},
     pin::Pin,
+    rc::Rc,
     sync::Arc,
     task::{self, Poll},
     time::{Duration, Instant},
@@ -174,7 +175,7 @@ impl InlineAssistant {
         if let Some(editor) = item.act_as::<Editor>(cx) {
             editor.update(cx, |editor, cx| {
                 editor.push_code_action_provider(
-                    Arc::new(AssistantCodeActionProvider {
+                    Rc::new(AssistantCodeActionProvider {
                         editor: cx.view().downgrade(),
                         workspace: workspace.downgrade(),
                     }),

crates/assistant/src/slash_command.rs

@@ -7,11 +7,13 @@ use editor::{CompletionProvider, Editor};
 use fuzzy::{match_strings, StringMatchCandidate};
 use gpui::{AppContext, Model, Task, ViewContext, WeakView, WindowContext};
 use language::{Anchor, Buffer, CodeLabel, Documentation, HighlightId, LanguageServerId, ToPoint};
-use parking_lot::{Mutex, RwLock};
+use parking_lot::Mutex;
 use project::CompletionIntent;
 use rope::Point;
 use std::{
+    cell::RefCell,
     ops::Range,
+    rc::Rc,
     sync::{
         atomic::{AtomicBool, Ordering::SeqCst},
         Arc,
@@ -322,7 +324,7 @@ impl CompletionProvider for SlashCommandCompletionProvider {
         &self,
         _: Model<Buffer>,
         _: Vec<usize>,
-        _: Arc<RwLock<Box<[project::Completion]>>>,
+        _: Rc<RefCell<Box<[project::Completion]>>>,
         _: &mut ViewContext<Editor>,
     ) -> Task<Result<bool>> {
         Task::ready(Ok(true))