assistant2: Add rudimentary chat functionality (#21178)

This PR adds in rudimentary functionality for sending messages to the
LLM in `assistant2`.

<img width="1079" alt="Screenshot 2024-11-25 at 1 49 11 PM"
src="https://github.com/user-attachments/assets/5accb749-c034-4fb2-bf55-3ae5bc9529ad">

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -1,5 +1,6 @@
 mod assistant_panel;
 mod message_editor;
+mod thread;
 
 use command_palette_hooks::CommandPaletteFilter;
 use feature_flags::{Assistant2FeatureFlag, FeatureFlagAppExt};
@@ -7,7 +8,10 @@ use gpui::{actions, AppContext};
 
 pub use crate::assistant_panel::AssistantPanel;
 
-actions!(assistant2, [ToggleFocus, NewThread, ToggleModelSelector]);
+actions!(
+    assistant2,
+    [ToggleFocus, NewThread, ToggleModelSelector, Chat]
+);
 
 const NAMESPACE: &str = "assistant2";