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/test-utils/test-utils.tsx

@@ -374,6 +374,7 @@ export function createMockFlipperLib(options?: StartPluginOptions): FlipperLib {
     },
     selectPlugin: jest.fn(),
     writeTextToClipboard: jest.fn(),
+    openLink: jest.fn(),
     showNotification: jest.fn(),
   };
 }