Feature Request: Add `hcl` format

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](https://developer.hashicorp.com/terraform/language/functions/yamldecode) and [jsondecode](https://developer.hashicorp.com/terraform/language/functions/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

The issue names no files, tests, or entry points. Start by reviewing issue #59 and Pkl's existing JSON and YAML output behavior; done means direct HCL output suitable for Terraform .tfvars files without intermediary conversion.

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