Implement the UnliftedDatatypes extension

GHC Proposal: 0265-unlifted-datatypes.rst
Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/265
Issues: #19523
Implementation Details: Note [Implementation of UnliftedDatatypes]

This patch introduces the `UnliftedDatatypes` extension. When this extension is
enabled, GHC relaxes the restrictions around what result kinds are allowed in
data declarations. This allows data types for which an unlifted or
levity-polymorphic result kind is inferred.

The most significant changes are in `GHC.Tc.TyCl`, where
`Note [Implementation of UnliftedDatatypes]` describes the details of the
implementation.

Fixes #19523.