feat: auto install when using CDN (#403)

Author: antfu

src/index.ts

@@ -1,6 +1,5 @@
 import Vue, { VueConstructor } from 'vue'
 import { Data, SetupFunction, SetupContext } from './component'
-import { currentVue } from './runtimeContext'
 import { install } from './install'
 import { mixin } from './setup'
 
@@ -10,17 +9,12 @@ declare module 'vue/types/options' {
   }
 }
 
-const _install = (Vue: VueConstructor) => install(Vue, mixin)
-const plugin = {
-  install: _install,
-}
-// Auto install if it is not done yet and `window` has `Vue`.
-// To allow users to avoid auto-installation in some cases,
-if (currentVue && typeof window !== 'undefined' && window.Vue) {
-  _install(window.Vue)
+const VueCompositionAPI = {
+  install: (Vue: VueConstructor) => install(Vue, mixin),
 }
 
-export default plugin
+export default VueCompositionAPI
+
 export { nextTick } from './nextTick'
 export { default as createElement } from './createElement'
 export { SetupContext }
@@ -40,3 +34,8 @@ export * from './apis/lifecycle'
 export * from './apis/watch'
 export * from './apis/computed'
 export * from './apis/inject'
+
+// auto install when using CDN
+if (typeof window !== 'undefined' && window.Vue) {
+  window.Vue.use(VueCompositionAPI)
+}