Extend the behavior of DecoratedCodeBlock

[cscheid]

DecoratedCodeBlock exists in 1.3 to solve a fairly narrow problem.

Concurrently, there's now a feature request for code folding in custom formats (confluence specifically). Our filter chain makes it hard for code folding to be detected in custom formats because they're done in a specific post- filter.

If we added code folding to the set of things that DecoratedCodeBlock is responsible for, then custom writers could handle it as well.

[cscheid]

See #4660