Add a way to toggle new swimming mechanics.

There needs to be a way to disable the new swimming mechanics that came with 1.13. A lot of people consider this unfair, and there is a very large number of servers that run 1.8 while supporting higher versions, making anyone using 1.13 or higher have an extremely unfair advantage in water.

One way I'd suggest this is a swimming gamerule that, when set to false, prevents players from swimming.