To be used by completions to directly determine whether it is either
possible or preferable to complete a switch (instead of a subcommand),
(presuming that switches must come before subcommands).
* __fish_can_complete_switches: we are in a position where a switch may
be placed.
* __fish_should_complete_switches: we're in a position to accept a
switch and the current token starts with `-` so we have no choice but
to do so.
