fix: single es-module-shims per page, embedded instead of a cdn request

The shim must be a page-wide singleton (in jupyterlab every federated
extension bundle would otherwise execute its own copy), so it stays a
global script tag - the tag doubles as the cross-library mutex: the DOM
check and the append run synchronously, exactly one library injects it,
others (e.g. ipyvue) wait for the importShim global. Previously only a
module-scoped flag was checked, so vue-first load order put two shims
on the page.

The tag's code is now embedded (same v1.8.2 ipyvue embeds, regeneration
one-liner in the file header) - no cdn request at runtime.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: maartenbreddels