ci(danger): add dangerjs for GitHub

Add GitHub workflow for running dangerjs on pull requests.
Add GitHub layout for DangerJS.

.github/dangerjs/prTargetBranch.ts

@@ -0,0 +1,19 @@
+import { DangerDSLType, DangerResults } from "danger";
+declare const danger: DangerDSLType;
+declare const fail: (message: string, results?: DangerResults) => void;
+
+/**
+ * Check if the target branch is "master"
+ *
+ * @dangerjs FAIL
+ */
+export default function (): void {
+    const prTargetBranch: string = danger.github?.pr?.base?.ref;
+
+    if (prTargetBranch !== "master") {
+        return fail(`
+        The target branch for this pull request should be \`master\`.\n
+        If you would like to add this feature to the release branch, please state this in the PR description and we will consider backporting it.
+        `);
+    }
+}