feat: format the current directory by default

[zimbatm]

The common case is to format folders, not stdin.

This changes the default to format the current directory, and introduces
a --stdin flag for users that want to wrap nixfmt into editors and
other tools.

See also NixOS/nix#11438

[github-actions]

Nixpkgs diff

[infinisil]

It looks like you're unaware of #240, where we deprecated directory recursing in favor of treefmt taking care of that. I'm not yet 100% sure on the interactions with your PRs 🤔

[zimbatm]

Indeed. With that direction in mind, I think we can drop this PR 👍

[Kamillaova]

It looks like you're unaware of #240, where we deprecated directory recursing in favor of treefmt taking care of that. I'm not yet 100% sure on the interactions with your PRs 🤔

imagine spawning millions of processes instead of one 😍

[piegamesde]

Please save your snark for elsewhere. Nixfmt of course still accepts passing multiple files via CLI arguments, as is common practice in such applications. It is up to the caller to do the right thing.

[infinisil]

Well said @piegamesde, thanks

Also, last time I touched treefmt, it only used a single process per 1024 files. It went through a rewrite by now, so I'm not sure if that's still the case, but if it's not that sounds like something to fix.

[Kamillaova]

Okay, thanks
If treefmt used one process before, I think it works the same way now