Reenable more verbose printing of stack

[cpp977]

Description

The method stan::var::print_stack() does currently only print the size of the stack. This was different in the past. It would be very handy to have more verbose information for debugging the reverse mode call stack.

Expected Output

I have no strong opinion in what exactly should be printed and especially, I can not judge what is doable. But since the behavior was different in the past one may can restore a similar behavior.

Related discourse:

https://discourse.mc-stan.org/t/current-state-of-stan-print-stack/26248

Current Version:

v4.3.0