Move JS source from packs to entrypoints (#30037)

app/javascript/entrypoints/share.jsx

@@ -0,0 +1,32 @@
+import './public-path';
+import { createRoot } from 'react-dom/client';
+
+import { start } from '../mastodon/common';
+import ComposeContainer  from '../mastodon/containers/compose_container';
+import { loadPolyfills } from '../mastodon/polyfills';
+import ready from '../mastodon/ready';
+
+start();
+
+function loaded() {
+  const mountNode = document.getElementById('mastodon-compose');
+
+  if (mountNode) {
+    const attr = mountNode.getAttribute('data-props');
+
+    if (!attr) return;
+
+    const props = JSON.parse(attr);
+    const root = createRoot(mountNode);
+
+    root.render(<ComposeContainer {...props} />);
+  }
+}
+
+function main() {
+  ready(loaded);
+}
+
+loadPolyfills().then(main).catch(error => {
+  console.error(error);
+});