style: Add basic @container rule parsing and boilerplate

For now parse a MediaFeatureCondition. That needs being made more
specific, but that is probably worth its own patch.

Differential Revision: https://phabricator.services.mozilla.com/D143192

components/style/stylist.rs

@@ -2810,6 +2810,7 @@ impl CascadeData {
                 CssRule::Style(..) |
                 CssRule::Namespace(..) |
                 CssRule::FontFace(..) |
+                CssRule::Container(..) |
                 CssRule::CounterStyle(..) |
                 CssRule::Supports(..) |
                 CssRule::Keyframes(..) |