Add additional flexibility on DotShape

[jklein24]

In discussions with folks about future features, it seems like it would be cool to add custom DotShapes via something like:

drawDot: DrawScope.() -> Unit

and maybe even

interface QrDotDrawScope : DrawScope {
  val dotPosition: IntOffset
  val dotColor: Color
}

This would even allow people to customize dot color and shape by position of the dot. Not sure how that'll affect scanner reliability, but could do some pretty cool stuff!