Custom blocks: copy & paste, markdown, html

[YousefED]

We want to write our own serializer to handle clipboard copy / paste. Possibly, we can also re-use this for the export to markdown / html functions

[davidchappy]

Hey folks, thanks for your work on this great library.

Would this issue handle pasting images into the editor (via right-click -> copy image in browser, for example) without additional customization?

I've experimented with this approach, but I've not found a way to customize BlockNote's existing Image block.