Move "quilla_prevalidate" hook

[cryptaliagy]

quilla_prevalidate can also be put in its own file to handle more generic cases.

Originally posted by @yucui-avengers in #19 (comment)

Once more hooks are created, it probably won't make sense to keep "quilla_prevalidate" in the configuration hookspec file, since it isn't exactly a configuration hook (but it also is not not a configuration hook, in a way).