Add OpenTofu, use by default for Terraform (#391)

Seems fairly straightforward to me, Terraform is no longer open-source,
OpenTofu is the compatible open-source successor, similar to the
Redis/Valkey situation.

Of course, if someone still prefers to use Terraform instead, they can
easily customize using:

```tf
(setf (alist-get 'terraform-mode apheleia-mode-alist) 'terraform)
```

We don't have a mechanism for allowing multiple formatters to be
registered for the same major mode, where later ones could be used if
earlier ones are not installed. That might be a nice addition.

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