Get project roots for support relative paths for JSX source in React DevTools

See https://github.com/facebook/react-devtools/pull/703, it would be better for `Show <> in Source` feature for React DevTools.

- Make [`react-native-debugger-open`](https://github.com/jhen0409/react-native-debugger/tree/master/npm-package) can detect `projectRoots`
- It will not work if debugger is already connect to packager
- Use `REACT_DEBUGGER` env with [`react-native-debugger-open`](https://github.com/jhen0409/react-native-debugger/tree/master/npm-package) for RN packager, it will included [`projectRoots`](https://github.com/facebook/react-native/blob/118e981174ae513426e2f5ddd7ee3c5860519ff7/local-cli/server/middleware/getDevToolsMiddleware.js#L32)

Also, it's possible to set a function to debugger worker, let user set the `projectRoots` in JS:

```js
if (window.setProjectRootForRNDebugger) {
window.setProjectRootForRNDebugger('')
}
```

Except the global function, it seems to be no good way without `rndebugger-open`, so we should make it `recommended to use` in the documentation.