feat: add treefmt_nix to builtins

[Lassulus]

depends on numtide/treefmt-nix#243

not sure if there is a policy before creating a builtin. but I didn't want to break usage with the treefmt plugin. treefmt-nix allows configuring treefmt with nix and there is an optional wrapper that can be included in a devshell. this builtin is expected to be used together with devenv and the treefmt-nix wrapper inside the devshell.

@Mic92 maybe has some feedback on this approach :)

[mochaaP]

Is it possible for you to use .with() for this? Some integration might detect the diagnostics source and this will break them with this approach.

[jfly]

I'm excited about this! But the direction smells a little off to me: there's already a treefmt entrypoint with the config embedded, it's nix fmt.

IMO, we should instead be adding a generic "run nix fmt" builtin for none-ls. I've been working on that in this none-ls fork: main...jfly:none-ls.nvim:add-nix-fmt, but it requires a change to how treefmt's --walk=auto behaves so it'll be willing to format gitignored files passed explicitly on the command line.