What do you call extended words like ‘Nooooo’?

It's called (monosyllabic) word elongation or word lengthening.

From The Atlantic Magazine:

Evvvvverywherrrre, from instant messages to texts to tweets and even e‑mails, I see examples of what language watchers call word lengthening. The habit began among teens and 20-somethings, but it is no longer limited to them. Adults are adding o’s to their no’s, s’s to their yes’es, and i’s to their hi’s, to say nothing of a glut of exclamation points. In response to some recent news, my 60-something mom wrote, “LOVE IT AND YOU TOO!!!!” What is going on?

References:

• The Atlantic magazine
• Urban Dictionary
• ACL web
• Wired