LibWeb: Implement MediaQuery matching :^)

Currently, `evaluate()` recalculates whether the MediaQuery matches or
not, and stores it in `m_matches`, which users can query using
`matches()`. This allows us to know when the match-state changes, which
is required to fire MediaQueryList's change event.

Userland/Libraries/LibWeb/CSS/MediaList.h

@@ -29,6 +29,9 @@ public:
     void append_medium(String);
     void delete_medium(String);
 
+    bool evaluate(DOM::Window const&);
+    bool matches() const;
+
 private:
     explicit MediaList(NonnullRefPtrVector<MediaQuery>&&);