Using JS components cause layout shift

This MR fixes the layout shift entirely within the XB canvas, because the canvas does the 2 iframe trick where the iframe isn't swapped until the page load event fires, and adding the script tags delays that page load (which script tags do even without blocking=render).

This MR doesn't quite fix the layout shift on the published site and documents why in the code comment. I think it's worth merging this first (since it improves the in-XB editing experience), and then investigating the @todo separately.

In addition to fixing the layout shift, this also fixes a bug with not being able to drag into the slots, so bringing it into the current sprint.

Letting @larowlan decide when to merge based on https://git.drupalcode.org/project/experience_builder/-/merge_requests/7...

@effulgentsia: see that comment thread, @larowlan was suggesting an optimization that would only work per-frame, and would not pre-load ahead of time. Slightly different than what I suggested on the call earlier :)

Either way: I don't think those optimizations are blocking, but they do merit a follow-up.

Merged to 0.x. I'll open a follow-up for the further optimizations suggested in the MR reviews.

To ensure we don't forget #8.

All follow-ups were followed up … here!

Automatically closed - issue fixed for 2 weeks with no activity.