An increasing number of tourists are venturing to Thailand on trips centered around the culinary arts.  Some experts now say that Bangkok is fast gaining on Hong Kong, Singapore and Tokyo as a major destination for dining in Asia.   "Almost every cuisine is available [in Bangkok]," said Cheryl Tseng, author of the bar and restaurant guide Chic Bangkok.  "I think the scene is very metropolitan and international.  It has become a dining destination in Asia."  Travel agencies, including Peregrine Adventures of Australia, are now beginning to offer specialized tours of Thailand for food lovers.