Add wp-init and other small adjustments

[luisherranz]

What

Add data-wp-init and other small adjustments for this: WordPress/gutenberg#49868

Why

We need a basic version of wp-init, and there are some things that we won't need in Gutenberg.

How

For now, just add the wp-init copying wp-effect but using useEffect with an empty dependency array instead of useSignalEffect. Also add return's to be able to execute callbacks when the elements are removed.

I also removed the DOMContentLoaded event because we will use defer or type="module" in Gutenberg.

I also removed some unnecessary exports.

[DAreRodz]

LGTM! 👍