Styles: Add HiDPI / Retina Mode

[ItsJonQ]

In collaborating with @jasmussen , I realized that we needed to fine tune values for hidpi/retina displays - specifically, adjusting a value from 2px (default) to 1.5px.

We can add support for this by adding a hiDpiMode to the Style System, just like how we handle the other modes (e.g. dark, high contrast).

These styles may need to render slightly differently, using a hi-dpi CSS media query rather than relying on an HTML attribute.