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

[erikerlandson]

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.