Percentage radius

I have a custom squircle painter that I 'stole' off flutter/flutter PRs a while back. One thing I did was add the ability to paint percentage radii. It's a super handy feature, and the only thing stopping me from switching to `figma_squircle` full time.

While there are ways to do this with a LayoutBuilder, there is enough information available in OutlinedBorder to paint percentage radii from a constructor and a percentage value.