LibWeb: Use a string instead of an internal Parser class in Supports

Now that we can serialize CSS tokens, we can just hold a string and then
re-parse it when the Supports is evaluated. This feels a little weird,
but it only happens once so it's not going to slow it down much, and it
keep the API cleaner.

Userland/Libraries/LibWeb/CSS/Supports.h

@@ -21,8 +21,7 @@ class Supports final : public RefCounted<Supports> {
 
 public:
     struct Feature {
-        // FIXME: Using this internal parser class is a bit of a hack.
-        StyleDeclarationRule declaration;
+        String declaration;
         MatchResult evaluate() const;
     };