refactor: update plugin install for Vue 3 API

Author: yyc1217

src/index.js

@@ -4,13 +4,13 @@ import County from './components/counties.vue'
 import ZipcodeGroupby from './components/zipcodes.groupby.vue'
 
 // Declare install function executed by Vue.use()
-export function install(Vue) {
+export function install(app) {
 	if (install.installed) return;
 	install.installed = true
 
-	Vue.component('county', County)
-    Vue.component('zipcode', Zipcode)
-    Vue.component('zipcode-groupby', ZipcodeGroupby)
+	app.component('county', County)
+	app.component('zipcode', Zipcode)
+	app.component('zipcode-groupby', ZipcodeGroupby)
 }
 
 // Create module definition for Vue.use()
@@ -30,8 +30,9 @@ if (GlobalVue) {
 }
 
 // To allow use as module (npm/webpack/etc.) export component
+export default plugin
 export {
-    Zipcode,
-    County,
-    ZipcodeGroupby
+	Zipcode,
+	County,
+	ZipcodeGroupby
 }