feat: add contains method to List and Set classes with example and documentation

Add a `contains(element)` method to the built-in `List` class in `builtin/core.wren` and its embedded source in `src/wren_core.c`, enabling linear search for an element. Introduce a new `Set` class example in `example/set.wren` implementing set operations (union, intersection, difference) using the new `contains` method. Update `doc/site/classes.markdown` with a warning against inheriting from built-in types due to internal bit representation constraints.

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";