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/assistant2/src/inline_assistant.rs

@@ -45,6 +45,7 @@ use std::{
     iter, mem,
     ops::{Range, RangeInclusive},
     pin::Pin,
+    rc::Rc,
     sync::Arc,
     task::{self, Poll},
     time::Instant,
@@ -178,7 +179,7 @@ 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(),
                     }),