worktree: `but branch new --worktree new-branch`

[schacon]

We should have a way to create a worktree for a project that is not necessarily based off an existing branch in the workspace.

This would

❯ but status                                                                                                
╭┄00 [Unassigned Changes]
┊
┊╭┄t6 [sc-branch-29] {worktree} (no commits)
┊┊    (.git/gitbutler/worktrees/sc-branch-29)
├╯
┊
┊╭┄h4 [user-changes] (no commits)
├╯
┊
● 204e309 (common base) [origin/main] Merge pull request #10 from schacon/sc-description