Fix use mixin API feat continuous operation (#5610)

* Update use.js

fix:use feat continuous operation

* Update mixin.js

fix:mixin feat continuous operation

src/core/global-api/mixin.js

@@ -5,5 +5,6 @@ import { mergeOptions } from '../util/index'
 export function initMixin (Vue: GlobalAPI) {
   Vue.mixin = function (mixin: Object) {
     this.options = mergeOptions(this.options, mixin)
+    return this
   }
 }

src/core/global-api/use.js

@@ -6,7 +6,7 @@ export function initUse (Vue: GlobalAPI) {
   Vue.use = function (plugin: Function | Object) {
     /* istanbul ignore if */
     if (plugin.installed) {
-      return
+      return this
     }
     // additional parameters
     const args = toArray(arguments, 1)