Create v6 SimpleToast with Styled System

[TyMick]

Another #378— Here's a v6 SimpleToast that replaces the styleOverrides prop with Styled System utilities for color, layout, and position. I also replaced an old theme prop to use the new Styled System theme instead.

The most notable changes are in styled.jsx. In it, since the default styles include a media query, I placed the Styled System primitives inside an && block:

&& {
  ${color}
  ${layout}
  ${position}
}

This way, if a developer assigns a single value instead of an array to any of those Styled System props, meaning for that single value to apply across all screen width breakpoints, this && specificity will ensure it wins any conflicts with defaults inside in the media query.