Feature Request: Add hcl format

[garysassano]

While Pkl currently supports various formats like json and yaml, direct output in hcl would be a powerful addition for Terraform users.

Although generating HCL through intermediary formats like YAML or JSON is technically possible using HCL's yamldecode and jsondecode functions, it introduces unnecessary complexity. Direct HCL output would significantly streamline the workflow for creating .tfvars files used in Terraform configurations.

This feature would eliminate conversion steps, reduce potential errors, and solidify Pkl's position as a valuable tool within the Terraform ecosystem.

See also #59