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/__tests__/api.node.tsx

@@ -52,6 +52,7 @@ test('Correct top level API exposed', () => {
       "createDataSource",
       "createState",
       "createTablePlugin",
+      "getFlipperLib",
       "produce",
       "renderReactRoot",
       "sleep",