Embeds: Ensure that classic embed support works in the block editor.

See https://github.com/WordPress/gutenberg/pull/6345

Fixes #45447.

Props swisspidy, pento, audrasjb, aduth, jrchamp, thrijith, TimothyBlynJacobs, whyisjake. 
 

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git-svn-id: https://core.svn.wordpress.org/trunk@47904 1a063a9b-81f0-0310-95a4-ce76da25c4cd