feat: add Set class example and List.contains method to core library

Add a complete Set class implementation in example/set.wren with union, intersection, and set minus operations. Implement List.contains method in builtin/core.wren and its C source to support element existence checks via iteration.

src/wren_core.c

@@ -84,6 +84,15 @@ static const char* libSource =
 "    }\n"
 "    return result\n"
 "  }\n"
+"\n"
+"  contains(element) {\n"
+"    for (item in this) {\n"
+"      if (element == item) {\n"
+"        return true\n"
+"      }\n"
+"    }\n"
+"    return false\n"
+"  }\n"
 "}\n"
 "\n"
 "class Range is Sequence {}\n";