Intent to ship: prefers-contrast

[Emilio Cobos Álvarez]

In bug 1656363 I fixed up and enabled our implementation of the prefers
contrast media query by default.

This is implemented in Safari and Blink as well and complements the
already-existing forced-colors media query, to let pages react to high
contrast mode and so.

Bug: https://bugzil.la/1656363

Specification: https://drafts.csswg.org/mediaqueries-5/#prefers-contrast

Standards Body: W3C / CSSWG

Platform coverage: All

DevTools bug: N/A

Other browsers: Both Blink and WebKit implement this.

web-platform-tests: To the extent that this is testable on WPT, this is
tested: https://wpt.fyi/results/css/mediaqueries/prefers-contrast.html

Cheers,

-- Emilio