LibJS: Implement 'in' operator

Author: https://github.com/linusg
Commit: 11728b7db5
Pull-request: https://github.com/SerenityOS/serenity/pull/1931
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Runtime/Value.h

@@ -222,6 +222,7 @@ Value mod(Interpreter&, Value lhs, Value rhs);
 Value exp(Interpreter&, Value lhs, Value rhs);
 Value eq(Interpreter&, Value lhs, Value rhs);
 Value typed_eq(Interpreter&, Value lhs, Value rhs);
+Value in(Interpreter&, Value lhs, Value rhs);
 Value instance_of(Interpreter&, Value lhs, Value rhs);
 
 const LogStream& operator<<(const LogStream&, const Value&);