Block Editor: Add <base> to Block Canvas Iframe head

[vonloxx]

This allows relative paths to work inside a Block Canvas IFrame.

What?

Adding <base> with the current location to the blob used as a source for the Block Canvas Iframe.

Why?

In WordPress that's not an issue but on other projects using the Block Editor component it might be. It is an issue at Drupal Gutenberg.

How?

By adding<base>with current location, all urls will consider the url from that tag.

Testing Instructions

Not really an issue on WordPress but it can been seen at Drupal Gutenberg 3.0.0 Beta.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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[ellatrix]

This should fix the issue too right? #55152

[ellatrix]

I think it might be best to allow passing a head prop so you can add any HTML you want. The problem with this PR is that it assumes what the URLs are relative to. It might need a path as well, or maybe the front-end is hosted on a different domain.

[vonloxx]

Ah, tried to search for similar issue/PR but I missed that. Yes, this approach is not very flexible regarding paths or different domains.
The approach at #55152 seems a good solution.
I think that passing a head prop (if in block editor) won't work, for example, for Drupal Gutenberg because it uses the full Edit Post component 😬
I'll try #55152 and maybe close this later 😅

[Soean]

Merged in #66751