Bring `terraform-json` in as `tfjson` package

We have discussed some time ago that in pretty much all known cases downstream users use [`terraform-json`](https://github.com/hashicorp/terraform-json) in combination with `terraform-exec` (this library) and relatedly the two libraries are often updated in tandem, requiring a PR in two places to make a change involving JSON responses.

I'm therefore just noting down this discussion and the intention to move the library in here as part of this issue.

Start by reviewing the terraform-exec repository alongside the terraform-json repository linked in the issue, focusing on how the two libraries are currently maintained and used together. Define the migration scope and compatibility requirements before moving terraform-json into a tfjson package; done means the combined library supports the existing JSON-response use cases without requiring a separate repository change.

Written by the indexing model from the issue text.