Custom blocks: cleaning up & rerendering vanilla blocks

[YousefED]

Couple of leftovers from the Custom Blocks PR:

• for non-react custom blocks, there's no way to clean up event handlers (react blocks can do this with a normal useEffect
• for non-react custom blocks, there's no way to reuse the existing DOM, a new DOM is always created
• for both react and non-react, this issue prevents proper cleanup: https://discuss.prosemirror.net/t/nodeview-update-and-destroy-methods/5539/3, fixed in update packages incl prosemirror-view #223