asyncParseFn = (languageForExtension, languageForModuleSpecifier) =>
    mapParsers(
      languageForExtension,
      languageForModuleSpecifier,
      parserForLanguage,
      moduleTransforms,
      syncModuleTransforms,
    )
Async `mapParsers()` function; returned when a non-synchronous parser is
present _or_ when `moduleTransforms` is non-empty.

@type {MapParsersFn<AsyncParseFn>}