Add a new GitHub Actions workflow to track on Slack the new PRs merged into the WooCommerce repository (#39316)

Add a new GitHub Actions workflow to track on Slack the new PRs merged into the WooCommerce repository.

.github/workflows/test-assistant-pr-merge-track.yml

@@ -0,0 +1,26 @@
+name: Send a Slack notification when PR is merged
+on:
+    pull_request_target:
+        types: [closed]
+
+permissions: {}
+
+jobs:
+  send-slack-notification-when-pr-merged:
+    if: "${{ github.event.pull_request.merged }}"
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Wait 2 minutes for other labelling jobs to finish
+        run: sleep 2m
+        shell: bash
+      - name: Notify Slack
+        uses: archive/github-actions-slack@d9dae40827adf93bddf939db6552d1e392259d7d
+        id: notify
+        with:
+            slack-bot-user-oauth-access-token: ${{ secrets.TEST_ASSISTANCE_BOT_TOKEN }}
+            slack-channel: ${{ secrets.WOO_PR_MERGED_SLACK_CHANNEL }}
+            slack-text: |
+                <${{ github.event.pull_request.html_url }}|${{ github.event.pull_request.title }}>
+                *Labels:* ${{ join(github.event.pull_request.labels.*.name, ', ') }}
+            slack-optional-unfurl_links: false
+            slack-optional-unfurl_media: false