Support async generator functions and for-await statements

Created by: n3tr

Aim to resolve #4206 (closed)

Add @babel/plugin-proposal-async-generator-functions in babel config to support async generator functions and for-await statements

async function* agf() {
  await 1;
  yield 2;
}

Test steps

• yarn create-react-app my-agf-app
• cd my-agf-app
• Add async function* agf() { } to index.js
• yarn start
• App starts and works