Infer formatter from file extension by default

[olebedev]

Follow up issue to this twitter conversation we had with @domenkozar the other day.

Below is initial example that caught my attention by duplication of a file extension and formatter, they are essentially the same thing, but typed twice.

{ pkgs, lib, config, ... }: {
  files = {
    "foo.json".json = {
      foo = "bar";
    };
    "dir/foo.txt".text = "foo";
  };
}

I suggest to create a default heuristic so we infer formatters from the file extension and fall back to explicit definition when necessary. For example:

{ pkgs, lib, config, ... }: {
  files = {
    # Formatters inferred
    "foo.json" = {
      foo = "bar";
    };
    "dir/foo.txt"= "foo";

    # Formatter defined within configuration object, hence the `files` module 
    # must support polymorphism
    "crates/foo/Cargo.lock" = {
      content = { ... };
      formatter = "json";
    };
  };
}

I think it is doable to create a map between the supported formatters and file extensions. By this, we keep cognitive load low and don't cut off total control when it's needed.