new page: sync editor gutter with textarea scroll (#259)

internal/web/static/new.js

@@ -9,6 +9,8 @@ function updateGutter() {
   gutter.textContent = s;
 }
 content.addEventListener('input', updateGutter);
+// #259: the editor scrolls itself; keep the gutter's numbers in step with it.
+content.addEventListener('scroll', () => { gutter.scrollTop = content.scrollTop; });
 updateGutter();
 
 function toast(msg, kind) {