Add highlighting for Terraform (HCL) language

Would be great to have Terraform/HCL highlighting capabilities as on GitHub. Would be intuitive to have either `hcl` or `terraform` as the name.

Thanks!

The issue names no files, tests, or entry points. Start by locating the repository’s existing language definitions and registration tests, then compare their expected naming and highlighting behavior with Terraform/HCL examples. Done means code snippets are highlighted correctly through either the hcl or terraform name, with coverage matching the project’s existing conventions.

Written by the indexing model from the issue text.