neovim: editor integration using LSP/null-ls

[lbodor]

Hi,

https://github.com/jose-elias-alvarez/null-ls.nvim has recently added basic support for treefmt. I use it with some extra config as shown in jose-elias-alvarez/null-ls.nvim#1512.

[jfly]

As a slight variant of the above, I've added the nix_flake_fmt builtin to none-ls: https://github.com/nvimtools/none-ls.nvim/blob/90e4a27ccaa25979a6b732b9f06dfa43b54957b7/doc/BUILTINS.md#nix_flake_fmt. If you're using flakes, this is a nice way to configure your editor which doesn't even assume you're using treefmt.