style: [css-properties-values-api] Implement parsing and serialization for @property at-rule

Implemented behind the new properties-and-values pref.

Support for the CSSPropertyRule WebIDL interface is also added in this
patch, because until it's added, any attempt to access the rule using
the CSSOM would crash the browser.

Depends on D178268

Differential Revision: https://phabricator.services.mozilla.com/D178270

components/style/stylist.rs

@@ -2849,6 +2849,7 @@ impl CascadeData {
                     self.extra_data
                         .add_page(guard, rule, containing_rule_state.layer_id)?;
                 },
+                // TODO: Handle CssRule::Property
                 CssRule::Viewport(..) => {},
                 _ => {
                     handled = false;
@@ -3090,6 +3091,7 @@ impl CascadeData {
                 CssRule::Supports(..) |
                 CssRule::Keyframes(..) |
                 CssRule::Page(..) |
+                CssRule::Property(..) |
                 CssRule::Viewport(..) |
                 CssRule::Document(..) |
                 CssRule::LayerBlock(..) |