feat: give each command a short form

[lorenzolfm]

wt delete is the command you type in the middle of other work, and it is the longest name in the tool. Each command that you run more than once for a repository now has a short form.

Command	Short
wt share	wt sh
wt add	wt a
wt cd	wt c
wt ls	wt l
wt sync	wt s
wt delete	wt d
wt prune	wt p

wt init and wt clone get none. Each is short already, and you run it one time for a repository.

A short form is a command

The forms are visible aliases, so wt --help prints each one beside its command:

delete      Remove a worktree and delete its branch [alias: d]

is_command already reads the aliases of a subcommand, so a short form is a command in every place that the tool asks the question. It wins over a worktree with the same name, the long form wt cd d reaches a worktree named d, and wt add d warns that it made one.

The fish integration

The wrapper function chooses between a command that writes for the user and a command that prints a path for cd. That choice reads a list of names, so each short form of a command that writes for the user belongs in it. Without d in the list, wt d eng-1234 would lose its output to the path test and print nothing.

The completions give both spellings of each command, and __fish_seen_subcommand_from takes the short form too, so wt d <TAB> still offers the worktrees and wt a <TAB> still offers the branches.