Enable ES6 Modules for WordPress Themes JS

functions.php

//load as ES6
function load_as_ES6($tag, $handle, $source) {
  if('some-script-handle' === $handle) {
    $tag = '<script src="' . $source . '" type="module" ></script>';
  }
  return $tag;
}
add_filter('script_loader_tag', 'load_as_ES6', 10, 3);