Allow loading of the editor with non-block content

[dd32]

If the source textarea contains non-block HTML/text, currently the editor loads without any content in the editor.

This PR borrows the loader handler from this isolated editor example, but adopt the block check to look for <!-- wp: as a better notion of whether something is a block content or not.

https://github.com/Automattic/isolated-block-editor/blob/trunk/src/browser/index.js

This fixes editing old posts written pre-block-editor on the WordPress support forums.