Extract `ContextEditor` to `assistant_context_editor` (#23433)

This PR extracts the `ContextEditor` to the `assistant_context_editor`
crate.

As part of this, we have decoupled the `ContextEditor` from the
`AssistantPanel`.

There is now an `AssistantPanelDelegate` that the `ContextEditor` uses
when it needs to interface with the Assistant panel.

Release Notes:

- N/A