static buildFormatParser(fmt, options = {}) {
    const { locale = null, numberingSystem = null } = options,
      localeToUse = Locale.fromOpts({
        locale,
        numberingSystem,
        defaultToEN: true,
      });
    return new TokenParser(localeToUse, fmt);
  }
Build a parser for `fmt` using the given locale. This parser can be passed
to {@link DateTime.fromFormatParser} to a parse a date in this format. This
can be used to optimize cases where many dates need to be parsed in a
specific format.

@param {String} fmt - the format the string is expected to be in (see
description)
@param {Object} options - options used to set locale and numberingSystem
for parser
@returns {TokenParser} - opaque object to be used