Windows: Improve support for High Contrast Mode

[ItsJonQ]

@jasmussen Has done some work in Gutenberg to improve support for high contrast mode in Windows.

Example:
https://github.com/WordPress/gutenberg/blob/e259fd03b7ba4852ef6b01be06ad4f5c4a100eec/packages/components/src/form-toggle/style.scss#L49

I'm not too familiar with how to do this. It appears that we need to leverage the outline CSS property for this.
The thing I'm unsure about is outline-offset. Within the Gutenberg codebase, it seems to vary between 2px and -2px.

[jasmussen]

Border and outline become fully opaque. Background-color and box shadow are removed entirely.

The way to trick it is to leverage that even a transparent outline, becomes opaque. So you can make a 2px solid transparent outline around a button, and you have a windows 10 focus style, even if you use box shadow for the focus style in the normal mode.