Add support for size tweens (SizedBox + FractionallySizedBox)

[esDotDev]

It would be nice to be able to tween the actual size of things, probably using SizedBox and FractionallySizedBox internally:

Button().animate().width(begin: 40, end: 200);
Button().animate().size(begin: Offset.zero, end: Offset(50, 100);

Button().animate().widthPct(begin: .2, end: 1);
Button().animate().sizePct(begin: Offset.zero, end: Offset(1, 1));

[Suprndm]

+1
Would love this, because with Scale on a single axis, it mess-up the aspect when using round corners and text.