I did not get that welcomely awful feeling from the two films to which this series serves as a prequel, “It” (2017) and “It Chapter Two” (2019), both from the director Andy Muschetti. Which is why I’m happy, if that’s the right word, to report that the first scene of this first episode of “It: Welcome to Derry” is scarier and more disturbing than everything in the two movies combined. With Muschetti once again behind the camera for the premiere, he and the showrunners, Jason Fuchs and Brad Caleb Kane, serve up a perfect nightmare of mounting panic and terror. (Fuchs wrote the episode.)