Set up coveralls Jest coverage tracking on GitHub (#1392)

Summary:
This adds some pretty nice unit test coverage analysis for our codebase. Especially useful for external pull requests as a gentle nudge to make sure they don't regress the topline number.

To learn more, check out https://github.com/coverallsapp/github-action

Pull Request resolved: https://github.com/facebook/flipper/pull/1392

Test Plan:
{F245038134}

{F245038132}

https://coveralls.io/builds/32250084

Reviewed By: jknoxville

Differential Revision: D22664361

Pulled By: passy

fbshipit-source-id: 4dc1fc79677da41a9d3ea198224c5678ae156ad4

.github/workflows/nodejs.yml

@@ -29,8 +29,14 @@ jobs:
       run: yarn lint
       working-directory: ${{env.desktop-directory}}
     - name: test
-      run: yarn test
+      run: yarn test --coverage
       working-directory: ${{env.desktop-directory}}
+    - name: coveralls
+      uses: coverallsapp/github-action@v1.1.0
+      with:
+        github-token: ${{ secrets.GITHUB_TOKEN }}
+        base-path: ${{env.desktop-directory}}
+        path-to-lcov: ${{env.desktop-directory}}/coverage/lcov.info
     - name: build linux
       if: matrix.os == 'ubuntu-latest'
       run: yarn build --linux