ci(danger): add dangerjs for GitHub

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

.github/dangerjs/prCommitsTooManyCommits.ts

@@ -0,0 +1,19 @@
+import { DangerDSLType, DangerResults } from "danger";
+declare const danger: DangerDSLType;
+declare const message: (message: string, results?: DangerResults) => void;
+
+/**
+ * Check if pull request has not an excessive numbers of commits (if squashed)
+ *
+ * @dangerjs INFO
+ */
+export default function (): void {
+    const tooManyCommitThreshold: number = 2; // above this number of commits, squash commits is suggested
+    const prCommits: number = danger.github.commits.length;
+
+    if (prCommits > tooManyCommitThreshold) {
+        return message(
+            `You might consider squashing your ${prCommits} commits (simplifying branch history).`
+        );
+    }
+}