assistant: Extract ContextEditor and ContextHistory to their own modules (#23422)

This PR extracts the `ContextEditor` and `ContextHistory`
implementations into their own modules so that it's clearer which parts
depend on other constructs in the `assistant` crate.

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -2,6 +2,8 @@
 
 pub mod assistant_panel;
 mod context;
+mod context_editor;
+mod context_history;
 pub mod context_store;
 mod inline_assistant;
 mod patch;