fix: use ecmaVersion 2019 for now

vue-eslint-parser 6.0.5 (which is depended on by eslint-plugin-vue
6.0.0 & 6.0.1) depends on espree 5.x, thus does not support ecmaVersion
2020.

This commit can be reverted once this PR vuejs/eslint-plugin-vue#985
gets merged and released.

index.js

@@ -1,5 +1,8 @@
 module.exports = {
   extends: [
     require.resolve('eslint-config-standard')
-  ]
+  ],
+  parserOptions: {
+    ecmaVersion: 2019
+  }
 }