Make getFlipperLib generally available, and use it to decouple opening links from Electron

Summary:
This stack reduces our direct dependency on Electron, for example by exposing our own API to open links.

Also exposing `getFlipperLib` as API from `flipper-plugin`, so that these utility methods are available outside plugin contexts as well.

Reviewed By: timur-valiev

Differential Revision: D29661689

fbshipit-source-id: 0c0523326eeb0d9d8fbe3e03c4609327bb53596b

desktop/flipper-plugin/src/index.ts

@@ -37,7 +37,7 @@ export {createState, useValue, Atom} from './state/atom';
 export {batch} from './state/batch';
 export {
   FlipperLib,
-  getFlipperLibImplementation as _getFlipperLibImplementation,
+  getFlipperLib,
   setFlipperLibImplementation as _setFlipperLibImplementation,
 } from './plugin/FlipperLib';
 export {