Create v6 Slider with Styled System

[TyMick]

Next stop on #378.

Slider didn't require too much updating: the only styleOverrides property being used was backgroundColor, to fix the background of inactive track when the slider is placed over a non-white backround. I just swapped that out with Styled System's backgroundColor/bg.

Also, since the main Box component is already receiving all the excess props, I added ...Box.propTypes to Slider's propTypes.

Would it be helpful if I restructured the TrackParts so that the blue background gradient stops at the thumb (correctly proportioning the gradient for the new stop point, maybe using Chroma.js) and we just give the inactive part of the track bg="transparent"? That way developers wouldn't have to worry about explicitly changing the background color prop when using Slider over a non-white background. Or is a white default background there still preferred (or the additional color scale computation too expensive)?