I was about to write the same thing @Richard_Sather did after creating this example for you!
This does just what you want, but recognize that it’s quite a bit more complicated (not even worth it, really) to 1) automatically remove such emoji when the priority is downgraded, and 2) prevent repeated emoji, e.g., in the case of adding a 
when an attachment is added to a task. In these occasional cases, I just manually remove the extra emoji.
Thanks,
Larry