message: fix: update fields typing for the parital dictionary interface
diff: import { isCallable, merge, interpolate } from './utils';
  import { ValidationMessageTemplate } from './types';
  import { extend, RuleContainer } from './extend';
  -import { getConfig } from './config';
  interface PartialI18nDictionary {
  name?: string;
  messages?: { [k: string]: ValidationMessageTemplate };
  names?: { [k: string]: string };
  - custom?: { [k: string]: { [r: string]: ValidationMessageTemplate } };
  + fields?: { [k: string]: { [r: string]: ValidationMessageTemplate } };
  }
  interface RootI18nDictionary {
  