feat(compiler-core): switch to @babel/parser for expression parsing

This enables default support for parsing bigInt, optional chaining
    and nullish coalescing, and also adds the `expressionPlugins`
    compiler option for enabling additional parsing plugins listed at
    https://babeljs.io/docs/en/next/babel-parser#plugins.

packages/compiler-core/package.json

@@ -30,7 +30,8 @@
   },
   "homepage": "https://github.com/vuejs/vue/tree/dev/packages/compiler-core#readme",
   "dependencies": {
-    "acorn": "^7.1.0",
+    "@babel/parser": "^7.8.6",
+    "@babel/types": "^7.8.6",
     "estree-walker": "^0.8.1",
     "source-map": "^0.6.1"
   }