command: Add -check flag to fmt

[kylemcc]

This addresses #15304. There are a couple ways to solve this, but I chose to modify the fmt command with a -check flag that produces the desired behavior.

I chose not to put this functionality in github.com/hashicorp/hcl/hcl/fmtcmd, because that would have required a change to the API. That has broader implications. If the consensus is that said functionality would be useful outside of terraform fmt, I can move the logic to the hcl lib.

Alternatively, fmt could have been updated to alter its return code without the -check flag - like gofmt. However, as pointed out, that changes the current behavior which could have a downstream impact. Additionally, the default semantics of terraform fmt are different from gofmt. Namely, -write defaults to true with terraform fmt, but false with gofmt. Unless that default were changed (which seems like bad idea), you would need to run terraform fmt -write=false to achieve the requested effect. terraform fmt -check seems simpler and more intuitive to me.

[apparentlymart]

Thanks for working on this, @kylemcc! Sorry it took so long to get reviewed.

[kylemcc]

👍

[ghost]

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