Owning the planet’s biggest superyacht is so passé. Today, you’re nothing unless you dump cash into a private plane or two –  or 38, as King Rama X of Thailand recently revealed . And even then, you’d better hire the very best interior designer to kit it out to your perfect spec. Nearly anything goes, no matter how extravagant or bizarre the request — and at whatever the cost. Sarah Mespelt Larranaga, a specialist in private plane interior design, told The Post that “design and oversight” can