Warn on cabal format invocation

[jasagredo]

Just adding a warning seems like a good enough solution for now as we don't seem to want to deprecate the command. I think this doesn't deserve a changelog or update to the docs.

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[ulysses4ever]

this doesn't deserve a changelog or update to the docs.

I disagree. If our goal is to inform users, these are effective tools to do that.

[jasagredo]

I will add the changelog but I disagree. We shouldn't swamp ourselves in bureaucracy. This PR just emits a warning, it is extremely minimal.

[ulysses4ever]

This PR is small, but that's not the point. The point is that it's only logical to use release notes (and probably docs) to convey that cabal format is barely usable.

[jasagredo]

Let me know if the changelog entry looks good to you @ulysses4ever. In particular I don't think this is significant.

About documentation, the command is not mentioned in the documentation and for now I think that is OK. We don't want to give it more visibility than necessary, I think.

[ulysses4ever]

Brilliant, thank you!