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

[jhen0409]

See facebook/react-devtools#703, it would be better for Show <> in Source feature for React DevTools.

• Make react-native-debugger-open can detect projectRoots
  • It will not work if debugger is already connect to packager
• Use REACT_DEBUGGER env with react-native-debugger-open for RN packager, it will included projectRoots

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

if (window.setProjectRootForRNDebugger) {
  window.setProjectRootForRNDebugger('<path_of_project>')
}

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.