LibWeb: StyleResolver: Keep track of specificity of matched selector

This way it gets easier to compare matches.

Userland/Libraries/LibWeb/CSS/StyleResolver.h

@@ -18,6 +18,7 @@ struct MatchingRule {
     size_t style_sheet_index { 0 };
     size_t rule_index { 0 };
     size_t selector_index { 0 };
+    u32 specificity { 0 };
 };
 
 class StyleResolver {