Refactor UnliftedNewtypes-relation kind signature validity checks

This fixes three infelicities related to the programs that are
(and aren't) accepted with `UnliftedNewtypes`:

* Enabling `UnliftedNewtypes` would permit newtypes to have return
  kind `Id Type`, which had disastrous results (i.e., GHC panics).
* Data family declarations ending in kind `TYPE r` (for some `r`)
  weren't being accepted if `UnliftedNewtypes` wasn't enabled,
  despite the GHC proposal specifying otherwise.
* GHC wasn't warning about programs that _would_ typecheck if
  `UnliftedNewtypes` were enabled in certain common cases.

As part of fixing these issues, I factored out the logic for checking
all of the various properties about data type/data family return
kinds into a single `checkDataKindSig` function. I also cleaned up
some of the formatting in the existing error message that gets
thrown.

Fixes #16821, fixes #16827, and fixes #16829.