Is your feature request related to a problem? Please describe.
Stack only operates on selection-sized units, not block units, so //stack 1,0,0 is always a full selection in the X direction. This limits interesting stacking opportunities.

Describe the solution you'd like
//stack should have a new flag, -r (similar to //gen), that changes the offset units to blocks. It will be checked to ensure you do not stack inside of your selection, you must stack outside of the selection.

Describe alternatives you've considered
You can use alternating //stack and //move to shift things into place, but it is tedious.

Additional context
Forked from #1363.