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/Parser/Parser.cpp

@@ -1113,7 +1113,7 @@ Optional<Supports::Feature> Parser::parse_supports_feature(TokenStream<StyleComp
         TokenStream block_tokens { first_token.block().values() };
         if (auto declaration = consume_a_declaration(block_tokens); declaration.has_value()) {
             return Supports::Feature {
-                .declaration = declaration.release_value()
+                .declaration = declaration->to_string()
             };
         }
     }

Userland/Libraries/LibWeb/CSS/Parser/Parser.h

@@ -113,9 +113,6 @@ public:
 
     RefPtr<StyleValue> parse_as_css_value(PropertyID);
 
-    // FIXME: This is a hack, while CSS::Supports is using a StyleDeclarationRule
-    [[nodiscard]] Optional<StyleProperty> convert_to_style_property(StyleDeclarationRule const&);
-
 private:
     enum class ParsingResult {
         Done,
@@ -175,6 +172,7 @@ private:
 
     [[nodiscard]] RefPtr<CSSRule> convert_to_rule(NonnullRefPtr<StyleRule>);
     [[nodiscard]] RefPtr<PropertyOwningCSSStyleDeclaration> convert_to_declaration(NonnullRefPtr<StyleBlockRule>);
+    [[nodiscard]] Optional<StyleProperty> convert_to_style_property(StyleDeclarationRule const&);
 
     Optional<Color> parse_color(StyleComponentValueRule const&);
     Optional<Length> parse_length(StyleComponentValueRule const&);

Userland/Libraries/LibWeb/CSS/Supports.cpp

@@ -48,7 +48,7 @@ MatchResult Supports::InParens::evaluate() const
 
 MatchResult Supports::Feature::evaluate() const
 {
-    auto style_property = Parser({}, "").convert_to_style_property(declaration);
+    auto style_property = Parser({}, declaration).parse_as_declaration();
     if (style_property.has_value())
         return MatchResult::True;
     return MatchResult::False;

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;
     };