“The First Omen,” as it happens, is neither the first “Omen” (1976’s half-loved horror hit) nor the first “Omen” reboot (a misbegotten 2006 attempt). It’s not even this spring’s first movie about nuns in trouble and baby bumps in the night; that would be March’s “Immaculate” starring an unbound Sydney Sweeney, a film that compares favourably to this one for being crazier, gorier and ultimately more defiant. But “The First Omen” does have a certain swagger, like it was the only evil-pregnancy thriller in the world. Let’s credit debuting feature director Arkasha Stevenson (a former photographer …