feat(nixopts): display nix options in completion output

[Sporif]

- Annotate nix flags and introduce a custom usage function to display them in a dedicated section.
- Stop hiding nix flags so they show up in completion menus.
- Remove the default value text for StringMap options.
- Set a back-quoted name for StringMap options to replace the unclear default of "stringToString".

Though it makes discovering the available nix options much more natural, I'm still not entirely sure this is worth bloating the usage text and completions.

Edit:

• Stop hiding nix flags so they show up in completion output.
• Since they would otherwise be visible in the usage text, annotate
  nix flags and introduce a custom usage function to hide them from
  the usage text.

[water-sucks]

Displaying all the available Nix arguments in --help output seems like more noise than is needed for that specific section, considering how many options exist just for apply; based just off of vibes, I prefer --help being more terse and the actual available Nix options to be shown in the manpages and in completion output.

The only reason I agree with it being in completion output is for ease of use for the dynamic completion options (i.e. --update-input, etc.). Most of the completions don't actually share too many common prefixes past two letters besides options like --no-* and I think they will be inclined to consult the Nix manual anyway for these kinds of options if they are actually using them, perhaps?

[Sporif]

Ok, the completion output is the more important feature as it doesn't really have an alternative. I'll implement just that then.

[water-sucks]

This looks good to me :}