grammars.nix: allow the user to apply overlays

[mydumpfire]

You can now apply overlays to the grammar derivations via grammarOverlays. Also, the src in the derivation is now properly unpacked to the build directory, allowing the user to mutate the source files if they want to.

Example usage:

helix.override {
  grammarOverlays = [
    (final: prev: {
      markdown_inline = prev.markdown_inline.overrideAttrs {
        EXTENSION_TAGS = 1;
        EXTENSION_WIKI_LINK = 1;
        nativeBuildInputs = with pkgs; [ tree-sitter nodejs ];
        preBuild = ''
          rm src/parser.c
          tree-sitter generate
        '';
      };
    })
  ];
}

This builds the markdown_inline parser with optional extensions enabled.