feature: set up Firebase deploy

.github/workflows/firebase-hosting-pull-request.yml

@@ -0,0 +1,18 @@
+# This file was auto-generated by the Firebase CLI
+# https://github.com/firebase/firebase-tools
+
+name: Deploy to Firebase Hosting on PR
+'on': pull_request
+jobs:
+  build_and_preview:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - run: 'npm ci && npm run build:firebase:hosting'
+      - uses: FirebaseExtended/action-hosting-deploy@v0
+        with:
+          repoToken: '${{ secrets.GITHUB_TOKEN }}'
+          firebaseServiceAccount: '${{ secrets.FIREBASE_SERVICE_ACCOUNT_FB2ICAL_3051B }}'
+          projectId: fb2ical-3051b
+        env:
+          FIREBASE_CLI_PREVIEWS: hostingchannels