This PR creates a unified picker for all the Git-related pickers:
branch, worktree and stash. The main motivation for this is to bring
awareness and ease of access for the worktree and stash pickers that
were previously (and arguably) hidden amidst the sea of commands in the
command palette. As worktrees in particular become more relevant for
AI-related flows, having an easier way to reach for its picker will be
beneficial.

Note that the actions/commands remain the same; you can still look for
`git: worktree` or `git: view stash`. The difference is that these
actions will take you to the unified picker with the correct
corresponding active view.


https://github.com/user-attachments/assets/99d1cd6f-a19d-47d3-9bca-d7257e7ed5b8

Release Notes:

- Git: Unify the branch, worktree, and stash pickers into one, making it
easier to find and access them from one another.