Backport _wp_get_iframed_editor_assets

[ellatrix]

This includes the changes from:

• [Block editor: iframe: add enqueue_block_assets](​https://github.com/WordPress/gutenberg/pull/49655)
• [Post editor: iframe if all blocks are v3](​https://github.com/WordPress/gutenberg/pull/48286)

This includes the changes from:

• ​Block editor: iframe: add enqueue_block_assets
• ​Post editor: iframe if all blocks are v3

Trac ticket: https://core.trac.wordpress.org/ticket/58626

Looks like wp_print_fonts is missing. Are we waiting for other back ports?

so we might as well avoid introducing new ones in this PR and having to update the other one

@mukeshpanchal27 What do you mean?

so we might as well avoid introducing new ones in this PR and having to update the other one

@mukeshpanchal27 What do you mean?

[isabel_brison]

In 56047:

Editor: add iframe around post editor.

Adds an iframe around the post editor only if all registered blocks are v3 or higher.

Props ellatrix, mukesh27, costdev, oandregal, ramonopoly.
Fixes #58626.

Committed in r56047 / ​a7a32c3.