Call an expr while dubugging

[aaadddfgh]

Many debugging tools allow developers run an expr at a breakpoint.
Such as GDB can run call (int)printf("output when break at lines\n") or call someIntegerVar=10 when the program stops at a breakpoint. Will developers be able to do similar thing in Godot?

[Calinou]

Thanks for the proposal! Consolidating in #4473.