Move JS source from packs to entrypoints (#30037)

app/javascript/entrypoints/error.js

@@ -0,0 +1,14 @@
+import './public-path';
+import ready from '../mastodon/ready';
+
+ready(() => {
+  const image = document.querySelector('img');
+
+  image.addEventListener('mouseenter', () => {
+    image.src = '/oops.gif';
+  });
+
+  image.addEventListener('mouseleave', () => {
+    image.src = '/oops.png';
+  });
+});