assistant2: Factor out ContextStrip (#22096)

This PR factors a `ContextStrip` view out of the `MessageEditor` so that
we can use it in other places.

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -3,6 +3,7 @@ mod assistant_panel;
 mod assistant_settings;
 mod context;
 mod context_picker;
+mod context_strip;
 mod inline_assistant;
 mod message_editor;
 mod prompts;