Port comment character count functionality to TypeScript

app/javascript/retrospring/features/answerbox/comment/count.ts

@@ -0,0 +1,19 @@
+export function commentCharacterCountHandler(event: Event): void {
+  const input = event.target as HTMLInputElement;
+  const id = input.dataset.aId;
+  const counter = document.querySelector(`#ab-comment-charcount-${id}`);
+  const group = document.querySelector(`[name=ab-comment-new-group][data-a-id="${id}"]`);
+
+  if (group.classList.contains('has-error')) {
+    group.classList.remove('has-error');
+  }
+
+  counter.innerHTML = String(160 - input.value.length);
+  if (Number(counter.innerHTML) < 0) {
+    counter.classList.remove('text-muted');
+    counter.classList.add('text-danger');
+  } else {
+    counter.classList.remove('text-danger');
+    counter.classList.add('text-muted');
+  }
+}