Christopher Lowell is watching the beauty and heartbreak of humanity play out onstage in Marjorie Prime on Broadway, and there's nowhere he'd rather be. Written by Jordan Harrison and directed by Anne Kaufman, the drama follows Marjorie, an aging woman suffering from dementia who is spending her final months with a holographic replica"a "Prime""of her late husband, Walter. Played by Lowell, the Prime is a young, hand…