Implement Margin and fix Padding on Spices

At the moment the padding_* properties are happening after the styles are applied to the text, which means padding doesn't behave like you'd expect it to. If you add padding to something with background, the padded space doesn't have the background.

In the Spices.do_render private function there's a section where we're handling the padding, that would become about margin_* instead:

https://github.com/leostera/minttea/blob/main/spices/spices.ml#L120-L124

And then similar code for padding would be added earlier on that function, where we do the core rendering of the string.