feat: add mixins support

Author: Rostislav Khaniukov

lib/index.ts

@@ -1,4 +1,4 @@
-import Component from 'vue-class-component'
+import Component, { mixins } from 'vue-class-component'
 import { State, Getter, Action, Mutation, namespace } from 'vuex-class'
 
 Component.registerHooks([
@@ -14,4 +14,4 @@ Component.registerHooks([
   'validate'
 ])
 
-export { Component as default, State, Getter, Action, Mutation, namespace }
+export { Component as default, mixins, State, Getter, Action, Mutation, namespace }