There is a pizza emoji, I don't know when it was added but it's there in version 1.3.00.8663 just tricky to find. I can't type ":pizza:" because ":p" is already in use. I don't know if there's an easier way, but if I type :izza and then use the left arrow key to go back and add the p it works. Or you can try to type :pi extremely fast, or try to paste :pizza: from another app.
sohail rahman commented
Highly anticipated feature