Make Swift Sneak obtainable

Swift Sneak is an enchant that allows you to move faster while sneaking. This is potentially useful as a base-building tool (though adding flight in claims was mentioned in the past as being considered, so not sure if it will be important). I am also generally in favor of vanilla features being in play unless there is a reason to remove them. The only vanilla way to obtain it is from ancient cities. I don't think ancient cities generate due to the modified world height, so this enchant is most likely unobtainable. Given that it only affects sneaking speed, I don't think it should be an issue for PVP