Support color-scheme in Servo

[TimvdLippe]

With these changes, both the color-scheme CSS property is supported, as well as the required logic to act upon it.

Supersedes and closes #411

[mrobinson]

Is there a corresponding Servo PR for this change? Typically for Stylo PRs they should have a link to a Servo PR unless it is guaranteed to not change test results / affect the build.

[TimvdLippe]

It's still a draft as I ran out of time to create the Servo PR. Once I have that finished, I will mark this PR as ready

[TimvdLippe]

Stylo is currently being upgraded in servo/servo#46987 which means that test results will be muddied by the upgrade. Will wait on that

[Loirooriol]

I think this would be simpler as

Suggested change

	// If both are supported, then use the prefered color scheme to determine
	// whether the user wants dark mode
	if supports_dark_mode && supports_light_mode {
	return prefers_color_scheme == PrefersColorScheme::Dark;
	}
	// If only one is chosen, then it should only support dark mode if it was dark
	// mode enabled.
	if supports_dark_mode || supports_light_mode {
	return supports_dark_mode;
	}
	// Neither was chosen. In that case, fallback to the preferred color scheme
	return prefers_color_scheme == PrefersColorScheme::Dark;
	// If only one is supported, then use dark mode if it was the supported one.
	if supports_dark_mode != supports_light_mode {
	return supports_dark_mode;
	}
	// If either both or none are supported, then use the preferred color scheme
	// to determine whether the user wants dark mode.
	return prefers_color_scheme == PrefersColorScheme::Dark;