chore: send notification to Slack on release [skip ci] (#71)

I don't have a particularly good way of testing this.

Based on:
- https://github.com/cdklabs/construct-hub-webapp/pull/501/files

Using:
- https://github.com/bruceadams/get-release
- https://github.com/slackapi/slack-github-action

The webhook secret has already been added to the repo.

---

_By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license_

Author: Eugene Cheung

.github/workflows/release.yml

@@ -1,4 +1,5 @@
 const { awscdk, DependencyType, javascript } = require("projen");
+const { workflows } = require("projen/lib/github");
 
 const CDK_VERSION = "1.123.0";
 const CONSTRUCTS_VERSION = "3.3.69";
@@ -111,4 +112,42 @@ project.eslint.addRules({
 project.addPackageIgnore("*.ts");
 project.addPackageIgnore("!*.d.ts");
 
+project.release.addJobs({
+  notify_slack: {
+    name: "Send Slack notification",
+    runsOn: "ubuntu-latest",
+    permissions: {
+      actions: workflows.JobPermission.READ,
+    },
+    needs: [
+      "release",
+      "release_github",
+      "release_maven",
+      "release_npm",
+      "release_nuget",
+      "release_pypi",
+    ],
+    steps: [
+      {
+        name: "Get release",
+        id: "get_release",
+        uses: "bruceadams/[email protected]",
+        env: {
+          GITHUB_TOKEN: "${{ github.token }}",
+        },
+      },
+      {
+        name: "Send notification",
+        uses: "slackapi/[email protected]",
+        with: {
+          payload: `{"html_url": "\${{ steps.get_release.outputs.html_url }}", "tag_name": "\${{ steps.get_release.outputs.tag_name }}"}`,
+        },
+        env: {
+          SLACK_WEBHOOK_URL: "${{ secrets.SLACK_WEBHOOK_URL }}",
+        },
+      },
+    ],
+  },
+});
+
 project.synth();