Improve fish completions (#911)

* Fix fish completing for flag/option names that contain dashes.
* Fix fish completing for flag/option names that are `r`.
* Remove vestigial `present_flags_and_options` from fish completion scripts.
* Use double quotes for fish completion string test arguments.
* Simplify fish while loop.
* Make `FishCompletionsGenerator.swift` & generated fish completion scripts more concise.
* Require positional_index_comparison argument for all calls to fish shouldOfferCompletionsForPositionalFunction.

Signed-off-by: Ross Goldberg <484615+rgoldberg@users.noreply.github.com>