Add "where" command to the debugger

[d0c-s4vage]

Is your feature request related to a problem? Please describe

When debugging 010 templates with pfp, knowing the current stack trace within the template would be very useful. A where command, akin to pdb's where command, would be very useful.

Describe the solution you'd like

Add a where command to the debugger to print the current stack trace in the 010 template being executed.

Additional context

None