Add Hujsonfmt formatter

[fjij]

Add hujson formatter for tailscale's *.hujson format. It's primarily used for Tailscale ACL config files.

Testing: I use this branch in my own config: fjij/nix-configs@7bd05a4

Aside: I noticed someone else wanted to add this formatter as well, but this is PR is reduced in scope to only *.hujson files. #309

cc: @teto

[fjij]

@zimbatm

I'd like to request a review on this PR.

[zimbatm]

LGTM, thanks!