New gamerule: UndeadMobsBurn

This gamerule would make it so that Phantoms, Skeletons (and their variants), and Zombies (and their variants) wouldn't burn in the sunlight when this gamerule is set to false. I can see this being useful in a couple of ways. For example, maybe you want to make some sort of zombie apocalypse map, and zombies burning in the daylight makes it far too easy. Maybe you want to cure a Zombie Villager, and don't want to worry about it burning and dying before the conversion is finished. I think this should be one of the gamerules that you can toggle on/off without needing to activate cheats, just like things such as TNT explodes and fire spreads.