Corner rounding support

Add support for independant corner rounding using the rounding property:

const myBox = createBox({ rounding: [12, 12, 12, 12] });

Also add support for new anchor points, now 6 in total. Now in addition to each corner, you can size and position from edge centers.

type Anchor =
  | 'topLeft'
  | 'topCenter'
  | 'topRight'
  | 'bottomRight'
  | 'bottomCenter'
  | 'bottomLeft'
  | 'centerLeft'
  | 'center'
  | 'centerRight';