Neil Simon's relentless fertility is a real esthetic virtue. Clearly he can write a play about anything: it would be fun to set him up officially as America's playwright laureate, perhaps in the theater on Broadway that he owns, where he could create a continuous theatrical obbligato to the events of the day, dashing off a play on Billy Carter, or Lee Marvin's nonmarital problem or even a fast funny musical about the OPEC countries. And on the side he could run a more private service, whipping out personalized plays for ordinary people, say a one-acter on your kid's bar mitzvah or a gag epithalamium on your impending marriage. Something like this is what Simon has done in "They're Playing Our Song," apparently based on the real-life relationship between composer Marvin Hamlisch and lyricist Carole Bayer Sager, and which includes songs by Marvin Hamlisch and Carole Bayer Sager...