Add `$.on` vanilla JS substitute

app/javascript/retrospring/utilities/on.ts

@@ -0,0 +1,7 @@
+export function on(type: string, selector: string, callback: Function): void {
+  document.addEventListener(type, (event: Event) => {
+    if ((event.target as HTMLElement).matches(selector)) {
+      callback(event);
+    }
+  });
+};