Style-aware definition lists

[Omikhleia]

In conjunction with Omikhleia/markdown.sile#99

Rationale:
In some books, I'd need to be able to decide whether definition terms should be in boldface or not.
We probably need some variants (as for bullet/enum lists) as a document may need different strategies for different types of definition list, rather than a single global style.
But do we need a new style concept ("linked styles") to acknowledge a consistent choice between the term styling and the definition styling?