Recognizing that all consumers -- not just adults -- are spending more time online, toy company Mattel has created a virtual world for girls where they can create their own characters, design a room, shop with virtual money, play games, watch videos, have real-time chats with other girls and send e-mail.   The virtual world is called Barbie Girls, and it is being released in two stages.   The first is an online virtual world at www.barbiegirls.com, which launched last week. In July, Mattel, New York, will introduce a $59.99 MP3 player that, in addition to playing music, will include customizable fashions and accessories and serve as a key component to unlocking additional material on the Barbie Girls' Web site.   Barbie Girls is the result of listening to what girls want, researching how they play and fusing it with the right technology to deliver a completely new experience, Chuck Scothon, general manager and seni