Add suggestions for unrecognised pragmas

[ This is what happens if you talk to too many people about about wanting to contribute to GHC during ZuriHac :) ]

Fixes #21589 (closed) by adding a new hint, SuggestCorrectPragmaName, for suggesting possible corrections after a misspelled pragma (writing LNGUAGE instead of LANGUAGE, and the like).

This is not quite done, as I've never used alex and can't quite figure out how to do what I want, but I was assured by @sgraf812 that it's okay to post these WIP merge requests and ask for help.

TODOs:

• Fix tests referring to -Wunrecognised-pragmas (like parser/should_compile/read066), after figuring out the lexer part (see the comment below) as they now include more information. All of these have no possible suggestions, actually, so with !8451 (6040e130) this should be addressed.

R.e. some of the bullet point from the template:

have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)

I have added the fact that this closes #21589 (closed) to the commit description instead, but can of course change that and add it to the commit message directly if desired.