Import sortablejs only once (#25936)

Previously, `sortablejs` was imported twice, once synchronously and once
asynchronously, leading to webpack creating duplicate output code (once
in the index bundle, and once in a separate chunk). Fix this by always
asynchronously importing it. This was one of the build warnings observed
when trying to build with vite.

web_src/js/modules/sortable.js

@@ -0,0 +1,4 @@
+export async function createSortable(...args) {
+  const {Sortable} = await import(/* webpackChunkName: "sortablejs" */'sortablejs');
+  return new Sortable(...args);
+}