Add option to remove pragmas implied by the language variant

[mrBliss]

For example, by setting language_variant to GHC2021, all the extensions implied by the GHC2021 language variant will be removed as redundant language pragmas.

[mrBliss]

1. I was considering whether there should be a way to retain the former behaviour of remove_redundant: true, i.e., just remove BangPatterns and ViewPatterns when unused in the module. I decided not to add it because I couldn't think of a good way to make it fit the language_variant option (which language variant would that option be?) and the usefulness of the former behaviour is rather limited, IMHO.

2. What about projects that are mix of different language variants? The mix would be across components (library vs. testsuite, etc.)

[michaelpj]

This would be great to have! Could we also parse this setting out of the .cabal file like we do for default-extensions? I don't think that would directly solve your problem about different components with different languages (e.g. for default extensions it seems that we just take the union of all default extensions across all components!), but it's a step in the right direction.