⚡ improvement(flow,typescript): update missing handler typings

Author: kazupon

decls/i18n.js

@@ -48,7 +48,7 @@ declare type NumberFormats = { [key: Locale]: NumberFormat };
 declare type TranslateResult = string | LocaleMessages;
 declare type DateTimeFormatResult = string;
 declare type NumberFormatResult = string;
-declare type MissingHandler = (locale: Locale, key: Path, vm?: any) => void;
+declare type MissingHandler = (locale: Locale, key: Path, vm?: any) => string | void;
 
 declare type I18nOptions = {
   locale?: Locale,

types/index.d.ts

@@ -49,7 +49,7 @@ declare namespace VueI18n {
     interpolate(message: string, values?: Values): any[];
   }
 
-  type MissingHandler = (locale: Locale, key: Path, vm?: Vue) => void;
+  type MissingHandler = (locale: Locale, key: Path, vm?: Vue) => string | void;
 
   interface IntlAvailability {
     dateTimeFormat: boolean;