Add interface for excluding generated files from formatting

[roberth]

Current approach is somewhat cumbersome and is coupled to a specific formatting tool.

For example, config in the niv integration:

// lib.optionalAttrs (options ? pre-commit.hooks) {
      pre-commit.hooks.nixpkgs-fmt.excludes = [ "nix/sources.nix$" ];
    }

Suggested option

formatting.excludedFiles = mkOption {
  type = listOf (either str path);
}

• to be read by pre-commit (and any standalone formatters, unimplemented)
• to be written by tool modules that generate files, like niv
• also to be written by project-specific file generation (for example a nix file written by a terraform module)
• the str type is to be interpreted literally - no regex, globs etc

[roberth]

Use config.pre-commit.excludes = [ "some python regex" ].

This doesn't solve the problem for formatters outside pre-commit, but we'll solve that when it comes up.