allow specific formulae for linkapps

Per #35050. This PR allow users to be specific which formulae to link/unlink. e.g. brew (un)linkapps macvim. If no formula is given, it falls back to the old way. i.e. link/unlink all. Manpge has been updated accordingly. But we also need to update the shell completion if this PR is merged.
`If no <formulae> are provided, all of them will have their .apps symlinked.`