How talented is Emerald Fennell? So much so that in the span of two months she went tete-a-tete with Princess Diana as the indomitable Camilla Parker Bowles in Season 4 of “The Crown” and won the prestigious Best Screenplay honor from the Los Angeles Film Critics Association for her feature directorial debut, “Promising Young Woman.” And before the pandemic hit? Oh, she was just prepping a new “Cinderella” musical for the West End with some composer name Andrew Lloyd Webber.