Allow zero-length annotations via styled macro

[tecosaur]

It occurs to me that it could be useful to allow annotations in the styled"" macro that occur at a point/over an empty range, e.g.

styled"stuff{html=...:}more{html=...:}"

That said, these use cases could probably be refactored to something that would work with the current code, e.g.

styled"stuff{html_pre=...,html_post=...:more}`

I'm not sure whether this is worth doing, but it seemed worth opening an issue to record the thought.