RFE: support `mdoc:push` and `mdoc:pop` for scope

`mdoc:nest` seems close to "push a new nested scoping", but there does not seem to be a concept of "pop one scope up" - `mdoc:reset` basically pops all existing scope off.

I'd love a pair `mdoc:push` that starts a new nested scope, and `mdoc:pop` that moves up one scope layer, and only loses the current most-nested scope.