Intent to prototype: supports conditions for CSS imports

[CanadaHonk]

Summary:
Supports conditions in CSS imports allows CSS to be imported conditionally when a supports() condition is matched. With the initial patch (https://phabricator.services.mozilla.com/D172622), it will be enabled in Nightly by default.

Bug: https://bugzil.la/1427715
Specification: https://www.w3.org/TR/css-cascade-4/#conditional-import
Standards body: W3C CSSWG
Platform coverage: All

Preference: layout.css.import-supports.enabled
DevTools bug: https://bugzil.la/1827801
Standards position: https://github.com/mozilla/standards-positions/issues/761

Other browsers:
- Blink: Not shipped or implemented. Bug: https://crbug.com/1380321
- WebKit: Not shipped or implemented. No bug filed.

Web platform tests: https://wpt.fyi/results/css/css-cascade/import-conditions.html (and more added in Gecko patch, https://phabricator.services.mozilla.com/D172622)

How stable is the spec: CSS Cascading and Inheritance Level 4 is currently a W3C Candidate Recommendation Snapshot.