Make creating worktrees simpler and less error-prone

[stefanhaller]

Creating a worktree in lazygit used to be more awkward than it needed to be:

• The first thing you were asked was whether you wanted a "normal" or "detached" worktree — a distinction that's confusing to face up front, and meaningless when you're creating a worktree from a commit, tag or stash (both choices did the same thing there).
• You then had to type the new worktree's path by hand. That's easy to get wrong (a mistyped .worktrees, for instance), it wasn't clear whether you were meant to enter the parent folder or the full path, and it was ambiguous what a relative path would resolve against — especially when you were already inside another worktree.
• If you picked a branch that was already checked out in another worktree, lazygit still asked you for a path and only told you it wouldn't work afterwards.
• There was no quick way to create a new branch and a worktree for it (sharing the same name) in one step.

This PR reworks the whole flow:

• No more up-front mode menu. Pressing w on a branch, commit, tag, stash or remote branch opens a menu whose options are tailored to what you selected, and each option spells out exactly what it will do — e.g. "New worktree for 'main'", or "New detached worktree at 'v1.2.0'".
• You never type a path from scratch. Lazygit offers ready-made locations — the folders your existing worktrees already live in, plus an optional configured default — and shows each as a full path before anything happens. The folder name is filled in for you from the branch (or the name you choose). You can still pick "Other…" to type a custom path.
• Create a branch and its worktree in one step, with a single name used for both. Slashes are preserved, so feature/foo gives you a feature/foo branch and a matching nested folder.
• Branches already checked out elsewhere are caught up front: the relevant option is greyed out with an explanation of why, instead of letting you do the work and then failing.
• The worktrees panel's n is now a single branch picker. Start typing, or pick from the list: choosing an existing branch checks it out in a new worktree, choosing a remote branch creates a local tracking branch for it, and typing a new name creates a new branch off your current one — each then simply asks where to put the worktree. Branches that are already checked out are left out of the suggestions.

There's also a new worktree.defaultPath setting to tell lazygit where to create worktrees by default — useful before you have any existing worktrees for it to take its cue from. Starting that path with ~/ is supported (your home dir); this is also supported in the "Other…" path prompt mentioned above.

Fixes #3230
Fixes #4708
Fixes #5664