Support .tofu for Terraform lexing

[kivikakk]

Hi there! This PR adds support for lexing OpenTofu .tofu files as Terraform. They're the same format, and AFAICT have no different keywords.

For context:

• https://opentofu.org/docs/language/files/ — a little background on .tf vs .tofu and how OpenTofu treats them
• https://opentofu.org/docs/language/syntax/ — syntax reference
• Add OpenTofu .tofu extension to HCL github-linguist/linguist#7189 — GitHub linguist PR to add support for .tofu to existing Terraform support

[tancnle]

Thanks, @kivikakk. LGTM 🚀

[kivikakk]

Thanks @tancnle! 🤍