New feature request: cross ref for theorems, examples, and problems

[eraus]

Sometimes, we want to cross-reference theorems, examples, and problems. These can be referenced in the same way as equations---no caption is needed. We prefer to use the same syntax as equations. Please consider adding these features. If possible, please add a few more such as "user1", "user1", ..., so that users can customize their own.

[lierdakil]

It's been on the roadmap for quite a while, however pdf/latex is a bit of a problem. Latex is somewhat particular about what it can reference. I could perhaps add support for theorem-like things using \newtheorem, possibly with amsthm to improve compatibility with cleveref, but it's known to be wonky in edge cases.