Use terraform-json StateOutput for Output method return value

Currently, the Output method `terraform output -json` returns a map with its own type value i.e. `OutputMeta`

https://github.com/hashicorp/terraform-exec/blob/3fe5e310fd57434fdb99157c26d6dd1241c2b618/tfexec/output.go#LL39C73-L39C94

according to the project description, terraform-exec return value should use the data type defined in terraform-json which has the fit type for this case i.e. `StateOutput`

Start in output.go at the Output method and compare its current OutputMeta return type with terraform-json's StateOutput definition. Confirm that the method's public return value uses StateOutput for terraform output -json, with the existing behavior preserved.

Written by the indexing model from the issue text.