Their names are not perfect, so let's keep them as internal functions,
until we figure out how/if we want to expose this.
This reverts 0445126c2 (Undunder __fish_is_nth_token, 2021-06-29) (but I
did it without "git revert").
Closes#8008
Completions may benefit from using these in tandem to dynamically generate
completions predicated on the value of an earlier token in a cleaner fashion.
(Currently, most of called completion helper functions introspect the command
line to get the value of an earlier argument, making them less reusable for
different expressions that need completions of the same type. This way, the
completion can provide the function with the argument value explicitly.)
