LibCpp: Support parsing class declarations

Author: https://github.com/itamar8910
Commit: 4831dc0e30
Pull-request: https://github.com/SerenityOS/serenity/pull/6498

Userland/Libraries/LibCpp/Parser.h

@@ -69,6 +69,7 @@ private:
         Enum,
         Struct,
         Namespace,
+        Class,
     };
 
     Optional<DeclarationType> match_declaration_in_translation_unit();
@@ -80,6 +81,7 @@ private:
     bool match_expression();
     bool match_secondary_expression();
     bool match_enum_declaration();
+    bool match_class_declaration();
     bool match_struct_declaration();
     bool match_literal();
     bool match_unary_expression();