@spec get!(version :: String.t() | :current, boolean(), prefix :: String.t()) ::
  String.t() | no_return()

Gets the Magic: The Gathering Comprehensive Rules for the given version.

If the rules cannot be found locally, this attempts to download them from Wizards.

Versions are either a date written as "YYYYMMDD", or the atom :current. Note that the version does not necessarily match the effective date.

If the version is :current, the current version is obtained from Wizards, and the latest rules are downloaded.

If allow_online is false, any action that would go over the internet raises an error instead.

Downloaded rules are cached to prefix in addition to being returned.