Simplify format excluded files

The format exclusion list is quite long. A few things.

1. It doesn't need to exclude flake.lock, that shouldn't be being formatted at all.
2. Generated files could just go in gen/, and then we could just exclude gen/*. There could be gen/nix, gen/haskell etc.
3. If we do go ahead with that, horizon-gen-nix should write there by default
4. If we do go ahead with that, it might be working against us if we change any of the generated files to just be flake-parts modules, which is reasonable, but maybe that is better solved with symlinks anyway.

This might be quite a big task