Allow adding `|` as prefix for union

[sevenc-nanashi]

on TypeScript, this will not be error:

type voicevox_result_code =
  | "voicevox_result_succeed"
  | "voicevox_result_not_loaded_openjtalk_dict"
  | "voicevox_result_failed_load_model"
  | "voicevox_result_failed_get_supported_devices"
  | "voicevox_result_cant_gpu_support"
  | "voicevox_result_failed_load_metas"
  | "voicevox_result_uninitialized_status"
  | "voicevox_result_invalid_speaker_id"
  | "voicevox_result_invalid_model_index"
  | "voicevox_result_inference_failed"
  | "voicevox_result_failed_extract_full_context_label"
  | "voicevox_result_invalid_utf8_input"
  | "voicevox_result_failed_parse_kana"

but on rbs, it raises error:

type voicevox_result_code =
  | :voicevox_result_succeed
  | :voicevox_result_not_loaded_openjtalk_dict
  | :voicevox_result_failed_load_model
  | :voicevox_result_failed_get_supported_devices
  | :voicevox_result_cant_gpu_support
  | :voicevox_result_failed_load_metas
  | :voicevox_result_uninitialized_status
  | :voicevox_result_invalid_speaker_id
  | :voicevox_result_invalid_model_index
  | :voicevox_result_inference_failed
  | :voicevox_result_failed_extract_full_context_label
  | :voicevox_result_invalid_utf8_input
  | :voicevox_result_failed_parse_kana

It would be good if this is allowed.