This is just a refactor. I noticed that we now have a `project_core`
crate, which mainly contains the `Worktree` type and its private
helpers, plus the project's settings.
In this PR, I've renamed that crate to `worktree` and did some minor
simplification to its module structure. I also extracted a new
`WorktreeSettings` settings type from the `ProjectSettings`, so that the
worktree settings could live in the worktree crate. This way, the crate
is now exclusively about worktree logic.
Release Notes:
- N/A
