Add check for correct closing method of html element

[RoyaltyInTraining]

This addresses problems which multiple issues have mentioned (#432, #474).

The problem is: Block elements closed with ; and void elements closed with {} currently generate invalid HTML. Especially the block elements which are missing their closing tags can easily cause unexpected results after the browser attempts to silently fix the error.

This PR adds simple validation logic which emits errors when a mismatch is detected. It uses the current list of void elements in the HTML living standatd. The generation logic wasn't changed, so the output of valid usage remains identical. A html_unchecked macro is also included as an escape hatch.

[LnxgpI0k]

Does this enable optionally closing regular tags like void elements?

[RoyaltyInTraining]

Does this enable optionally closing regular tags like void elements?

No, it only helps developers to write correct code by highlighting mistakes. I know that some people want the added syntax, but since this is my first contribution to this project, I chose to not touch the ast module. I'm also not sure if the repo owner even wants this additional syntax rule, or if they prefer the current 1:1 mapping to html.