lang: add tofu to hcl file-types

[yu2971512385-ui]

OpenTofu 1.8 added the
.tofu extension so that a module can carry OpenTofu-specific code beside the
Terraform code it replaces: when foo.tofu and foo.tf both exist OpenTofu
loads only foo.tofu, and Terraform ignores .tofu entirely. A directory
written for OpenTofu alone therefore need not contain a single .tf file, and
today those files open with no language attached — no highlighting, no
terraform-ls.

Checked against OpenTofu 1.13.1 rather than taking the release notes on trust:

$ ls
main.tofu
$ tofu apply
Error: No value for required variable
  on main.tofu line 1:
   1: variable "who" {

The file is parsed and diagnostics point into it, so .tofu alone is a complete
configuration.

.tofuvars is deliberately left out: the RFC describes it as the replacement
for .tfvars, but 1.13.1 does not auto-load terraform.tofuvars, and with both
terraform.tfvars and terraform.tofuvars present the .tfvars value is the
one that wins. The .tf.json → .tofu.json replacement needs no entry either,
since those already match on json.

🤖 Generated with Claude Code