Docusarus: Build validation (#1329)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1329

Add build validation for `website-next`, also adding validation that `yoga-layout/sync` may be bundled by Webpack out-of-the-box.

Reviewed By: christophpurrer

Differential Revision: D47414176

fbshipit-source-id: 9adc4f9673333f4f79c91352f445489d6da05a07

.github/actions/setup-js/action.yml

@@ -17,4 +17,3 @@ runs:
     - name: yarn install
       shell: bash
       run: yarn install --frozen-lockfile
-      working-directory: javascript