react-refresh: add options to override $RefreshReg$ and $RefreshSig$ for better System.js integration

[vasiliicuhar]

Do you want to request a feature or report a bug?
Feature

Right now babel plugin emits globals: #16604

window.$RefreshReg$ = () => {};
window.$RefreshSig$ = () => type => type;

It would be nice to have them configurable. That would allow to use import.meta in environments like SystemJS and have simpler implementation:

import runtime from 'react-refresh/runtime'
runtime.injectIntoGlobalHook(window)

System.constructor.prototype.createContext = function (url) {
  return {
    url,
    $RefreshSig$: runtime.createSignatureFunctionForTransform,
    $RefreshReg$: (type, id) => {
      id = url + ' ' + id
      runtime.register(type, id)
    }
  };
};

If you don't mind I could create PR with changes to react-refresh/babel next week.

environment:

{
    "systemjs": "^6.1.4",
    "react": "^16.11.0",
    "react-dom": "^16.11.0",
    "react-refresh": "^0.6.0"
 }

[gaearon]

Happy to take a PR.