Add iFrame of the preview

Added a few small tweaks here to get this slightly closer to reality.

This actually kinda sorta almost works in the current state.

* Create a new basic page.
* The iframe will display a 404.
* Fill out necessary form fields.
* Click the preview button (you don't have to save the node)
* You'll navigate to what I'm now going to lovingly call 'different page preview'
* Click the 'back to content editing' button.
* You'll be back on the edit form, but the iframe will now contain the current preview.
* You can even change the view mode within the iframe to preview different view modes without navigating away from the form.

Depending on how this evolves, I think we'll have to remove the 'back to content editing' button from the iframe. But keeping it for now as it makes the process above possible.

Going to test this manually

Let's move the TODO about 10.1.0 above the whole Form API of html_tag to give a better indication of what would be replaced once we target 10.1

Also what is the dependency you are referring to on line 10?

Other than that, looks like a simple improvement. Approved.

During manual testing, the toolbar still seems to be included. Was the adminstrative toolbar supposed to be excluded?

Re #7 - I've seen that happen when clicking the 'Return to content editing' button in the iframe. At that point it is iframe inception and all bets are off :) If it isn't that, I'll have to try it out in Umami. I was testing in Olivero.

> Also what is the dependency you are referring to on line 10?

That was a note to say 'we should do this with dependency injection' rather that a static call. But that might not be possible in this procedural code at the moment.

What's missing from my screenshot was how it was displaying a "Page Not Found" page. That got cut off. So it's not actually showing the preview.

Propose that we RTBC this since we have an iFrame in the node edit form.

Merged and marking as fixed.

Surprised to see that it is actually possible to compile the preview for a node that hasn't even been created yet :) That simplifies a few things.