⚛️ Get rid of the <wp-block> wrapper

[DAreRodz]

This PR removes the <wp-block> wrapper and, instead, appends all wp block attributes to the element's attributes where blockProps are rendered.

The technique is a bit hacky because:

1. there is no way to render those attributes during save, as most of the required information is not available
2. attributes are appended to the block element modifying the saved content with grep_replace(), using a RegExp that could not be reliable enough.

[luisherranz]

Looks great to me. Looking forward to the day we have the WP_HTML_Walker available for these HTML injections 🙂

It may be worth noting that this would only work if we enforce blocks to have a single node wrapper. Maybe we need to add that to the requirements in the README.

[luisherranz]

Maybe we need to add that to the requirements in the README.

Added in 110dcbf.