A SizedBox with both scaled width and height.
Usage: Spacing.md.gapAll → SizedBox(width: 16.s, height: 16.s)
Spacing.md.gapAll
SizedBox(width: 16.s, height: 16.s)
SizedBox get gapAll { final v = _base * _s; return SizedBox(width: v, height: v); }