Decouple extension `Worktree` resource from `LspAdapterDelegate` (#20611)

This PR decouples the extension `Worktree` resource from the
`LspAdapterDelegate`.

We now have a `WorktreeDelegate` trait that corresponds to the methods
on the resource.

We then create a `WorktreeDelegateAdapter` that can wrap an
`LspAdapterDelegate` and implement the `WorktreeDelegate` trait.

Release Notes:

- N/A