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
This PR adds a new `assistant_context_editor` crate.
This will ultimately house the `ContextEditor` so that it can be
consumed by both `assistant` and `assistant2`.
For the purposes of this PR, we just introduce the crate and move some
supporting constructs to it, such as the `ContextStore`.
Release Notes:
- N/A
