This PR introduces a new crate for the code that will drive the V3 of the UI design, with the goal of sketching out the API that shows how to deal with uncommited files.

This sketch should also include an idea how to listen to changes to the filesystem to produce events that help maintain the current state.

Follow-up of #5912 .

Tasks

• initial state of uncommitted files
• a new CLI to show uncommitted files
• tests
  • case-insensitive FS test
  • also: see what happens if there are case-collisions (two files fold into one name on a case-insensitive fs)
• IMPORTANT: use gix from main before merging
  • executable bit + modified == modified? Doesn't matter, just be sure UI can easily show it.
  • untracked must be obvious
  • blend gix::*::Change::Type into Change::Modification maybe
    • Actually this helps the code, even though on the consumer side it's a little awkward to use
  • sort changes in tests
  • One set of information per path, worktree takes precedence and and overrides the index
    • ignore the index, but not untracked
  • no IntentToAdd for Frontend
  • Flag with TypeChange or +x or -x

Notes for the reviewer

• I left the code in but-core besides gix there are no dependencies needed to handle the but_core::WorktreeChanges.
• Definitely needs .git/index tracking - if it changes while a commit is made, we have to reconcile (or at least assure nothing can go wrong)

For later

• provide patch for a change
• locking information