Highlight upcoming modifications in debugger #3
Description
For example, if the cursor is on a +, we highlight the two uppermost elements on the stack (and optionally already print the result).
For a p command, we highlight the three uppermost elements on the stack and also the position in the playfield that will be modified.
In all cases, we can highlight the next field the cursor will visit (or a selection in case of ?).
It should be possible to disable this feature since the visual clutter might get overwhelming.