For usage in completion scripts.
Unlike `__fish_is_first_token` (which is probably not correctly named),
`__fish_is_first_arg` returns true regardless of whether existing tokens start with `-`
or not, to be used when an arg cannot be used with any other argument.
`__fish_prev_arg_in` is similar to `__fish_seen_...` but it explicitly
tests the preceding token only, for arguments that take only a single
parameter.
