Code from `spack.util.editor` was duplicated into our licensing hook in #11968. We
really only want one place where editor search logic is implemented. This consolidates
the logic into `spack.util.editor`, including a special case to run `gvim` with `-f`.
- [x] consolidate editor search logic in spack.util.editor
- [x] add tests for licensing case, where `Executable` is used instead of `os.execv`
- [x] make `_exec_func` argument of `editor()` into public `exec_fn` arg
- [x] add type annotations
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