Producers of a movie based on “Stealing Rembrandts,’’ the book about heists of works by the Dutch master at the Isabella Stewart Gardner Museum and elsewhere, have found their screenwriter. John Stimpson, a Worcester-area native whose credits include “Sexting in Suburbia’’ and “The Legend of Lucy Keyes,’’ has been tapped to write the screenplay for the movie based on the book by former Boston Herald reporter Tom Mashberg and art security expert Anthony Amore. “Stealing Rembrandts’’ tells the story of the Gardner heist and also the 1972 robbery of a Rembrandt (and a Paul Gauguin and a Pablo Picasso) from the Worcester Art Museum.