Build webpack stats and upload to Relative CI

This setup is in accordance with the workflow_run setup recommended by
RelativeCI for projects using a fork-based workflow.
https://relative-ci.com/documentation/setup/agent/github-action/#workflow_run-event

.github/workflows/build.yml

@@ -26,11 +26,16 @@ jobs:
         run: yarn install
       - name: Build artifacts
         run: |
+          yarn build:css:prod
           VERSION_FILE_NAME=$GITHUB_REPOSITORY \
           VERSION_FILE_VERSION=$(git describe --tags --exclude=latest) \
-          yarn build
-      - name: Upload
+          yarn build:js:prod --json webpack-stats.json
+      - name: Upload build
         uses: actions/upload-artifact@v1
         with:
           name: dist
           path: dist
+      - name: Upload webpack stats artifact
+        uses: relative-ci/agent-upload-artifact-action@v1
+        with:
+          webpackStatsFile: ./webpack-stats.json

.github/workflows/relative_ci.yml

@@ -0,0 +1,17 @@
+name: RelativeCI
+
+on:
+  workflow_run:
+    workflows: ["Build"]
+    types:
+      - completed
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Send webpack stats to RelativeCI
+        uses: relative-ci/agent-action@v2
+        with:
+          key: ${{ secrets.RELATIVE_CI_KEY }}
+          token: ${{ secrets.GITHUB_TOKEN }}