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

[radeksimko]

We have discussed some time ago that in pretty much all known cases downstream users use 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.