chore: esm external deps

Author: Guillaume Chau

build/rollup.config.es.js

@@ -6,6 +6,11 @@ const config = Object.assign({}, base, {
     file: 'dist/vue-virtual-scroller.esm.js',
     format: 'es',
   },
+  external: [
+    'scrollparent',
+    'vue-observe-visibility',
+    'vue-resize',
+  ],
 })
 
 export default config