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/slash_command.rs

@@ -1,4 +1,4 @@
-use crate::assistant_panel::ContextEditor;
+use crate::context_editor::ContextEditor;
 use anyhow::Result;
 use assistant_slash_command::AfterCompletion;
 pub use assistant_slash_command::SlashCommand;