Make GHC2021 extensions default

[xsebek]

• closes Consider using GHC2021 extensions by default #419

[xsebek]

This will likely cause merge conflicts, so we could consider leaving this as the last to merge 😉

[xsebek]

I wonder why Fourmolu failed in restyled? 🤔 It should find the extension in the cabal file just fine.

[byorgey]

No opinion on whether we should wait to merge this. We are going to have merge conflicts one way or another. I guess you mean if we wait to merge this, then maybe we will just be able to fix a bunch of merge conflicts here at once, instead of having to fix merge conflicts for every other pending PR.

On the other hand there are always going to be more PRs... 😄

[byorgey]

I wonder why Fourmolu failed in restyled? It should find the extension in the cabal file just fine.

Indeed, that is strange. https://hackage.haskell.org/package/fourmolu-0.7.0.1#language-extensions-dependencies-and-fixities does seem to imply this should work.

[xsebek]

Yes, it does, if you provide the --cabal-default-extensions flag. 😒

I am looking for a way to enable that in CI...

[byorgey]

Can we put it in fourmolu.yaml?

[xsebek]

No, but we should be able to add it to restyled.

[xsebek]

Hey @TristanCacqueray, would you happen to know how to fix restyled fourmolu?

I tried something in #521, but I could not get it to work with the 0.3.0.0 version. 😞