Change naming convention of gamerules to be like stats eg. entity.all.spawn

The benefits of this are kind of abstract, but I feel like it's easier to mentally hold all these gamerules in your mind if they follow a categorizing naming system, like scoreboard statistics, especially considering there will probably be many more gamerules in the future.

Given that the ability to toggle the spawning for individual mob types as been announced, and this will probably take the form of gamerules, I think now's the time to implement this superior naming convention.

/gamerule damage.fall false