This PR adds the ability to save in the settings whether new threads
should start in the current project or in a new Git worktree.
Additionally, it also adds a keybinding that allows cycling through the
menu options easily, with the ability to use cmd-click/enter to choose
which one is set as the default.

No release notes because this feature/settings depends on a feature flag
that isn't out yet.

Release Notes:

- N/A