Allow WARNING pragmas to be controlled with custom categories

Closes #17209 (closed). This implements GHC Proposal 541, allowing a WARNING pragma to be annotated with a category like so:

{-# WARNING in "x-partial" head "This is a partial function...." #-}

The user can then enable, disable and set the severity of such warnings using command-line flags -Wx-partial, -Werror=x-partial and so on. There is a new warning group -Wextended-warnings containing all these warnings. Warnings without a category are treated as if the category was deprecations, and are (still) controlled by the flags -Wdeprecations and -Wwarnings-deprecations.