Static schema support for Terraform Test and Mock files

[dbanck]

• Depends on: Static schema support for Terraform Test and Mock files terraform-schema#379

This PR adds Terraform Tests support. It adds the new "feature", registers it in the service handler, and introduces two new language IDs: terraform-test & terraform-mock.
It also updates hcl-lang to the latest main.

Some open TODOs (I bet there are more):

• Tests (postponed to later PRs)
• Load the matching Terraform module for test files (postponed to later PRs)
• Look through all TODO comment in the code
• Update terraform-schema version once that PR has been merged

[jpogran]