chore: add ci-notify-slack.yml (#46)

This PR adds `ci-notify-slack.yml` to send out slack notifications about
PRs. The content is copied from
[dfinity/stable-structures](https://github.com/dfinity/stable-structures/blob/c567ed2eb6f1754e676ce5ae2124b65b769d3498/.github/workflows/ci-notify-slack.yml).

Author: mraszyk

.github/workflows/ci-notify-slack.yml

@@ -0,0 +1,34 @@
+name: PR Slack Notification
+
+on:
+  pull_request:
+    types: [opened, ready_for_review]
+
+jobs:
+  notify-slack:
+    name: Notify Slack
+    if: github.event.pull_request.draft == false
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Sanitize PR title
+        id: sanitize
+        env:
+          RAW_TITLE: ${{ github.event.pull_request.title }}
+        run: |
+          ESCAPED_TITLE=$(echo "$RAW_TITLE" \
+            | sed 's/&/\&/g' \
+            | sed 's/</\&lt;/g' \
+            | sed 's/>/\&gt;/g')
+          echo "safe_title=$ESCAPED_TITLE" >> "$GITHUB_OUTPUT"
+
+      - name: Post to a Slack channel
+        uses: slackapi/slack-github-action@91efab103c0de0a537f72a35f6b8cda0ee76bf0a # v2.1.1
+        with:
+          method: chat.postMessage
+          token: ${{ secrets.SLACK_API_TOKEN }}
+          payload: |
+            channel: eng-execution-mrs
+            text: ":github: `${{ github.repository }}` <${{ github.event.pull_request.html_url }}|${{ steps.sanitize.outputs.safe_title }}>"