Add OpenTofu .tofu extension to HCL

[adalinesimonian]

Adds .tofu extension to HCL, as used by OpenTofu.

Description

OpenTofu supports .tf as used by Terraform, but uses .tofu to allow OpenTofu-specific behaviour to be ignored by Terraform. The project has also reserved any new OpenTofu syntax for compatibility reasons to .tofu files alone. New language-changing syntax that would require adjustments to syntax highlighting has not been introduced, so therefore I believe it is safe to simply add .tofu as an extension for HCL.

Adoption of .tofu seems to have been more rapid amongst end-users, who likely have private repositories. While there are still several hundred results on GitHub, most lengthy, good examples of HCL in use with the .tofu extension were likely going to be behind this wall. Therefore, I wrote a length-enough example that resembles code I have in production at my employer and with other projects. I am happy to have it licensed under Linguist's MIT licence.

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.tofu
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • Self-written, see above
    • Sample license(s):
      • MIT