A singer-songwriter of jingles made his fortune on a fast-food commercial, but now he wants to write his own musical and lacks the self-assurance to begin.Attending a local production, he is smitten with a young woman who is appearing in a minor acting role as a hotel maid. She also lacks self-confidence, and the two engage in an love affair as they try to convince the other that they “can make it.”She goes on to become a star, but he hits the gutter.This basically is the plot of the new musical “Darling …