Upgrade to ECMAScript 2020

This is needed in order to use `globalThis`, see #1894.
It also made it possible to remove some old polyfills and fallbacks.

tests/integration/multi-nodejs-test/.eslintrc.js

@@ -1,11 +1,4 @@
 module.exports = {
-  extends: ['eslint:recommended', 'prettier'],
-  globals: {
-    self: false
-  },
-  env: {
-    node: true
-  },
   rules: {
     'no-console': 'off',
     'no-var': 'off'