style: implement -moz-inert CSS property.

-moz-inert CSS property reflects inert subtrees concept and can be used to implement HTML:dialog element and HTML:inert attribute

Differential Revision: https://phabricator.services.mozilla.com/D81701

components/style/element_state.rs

@@ -145,6 +145,9 @@ bitflags! {
         ///
         /// https://html.spec.whatwg.org/multipage/#centered-alignment
         const IN_MODAL_DIALOG_STATE = 1 << 53;
+
+        /// https://html.spec.whatwg.org/multipage/interaction.html#inert-subtrees
+        const IN_MOZINERT_STATE = 1 << 54;
     }
 }