terraform: Fix `provider::terraform::*` function names

[wata727]

See also https://github.com/terraform-linters/tflint/pull/2030

I had overlooked https://github.com/hashicorp/terraform/pull/34830, so the names in development were being used as Terraform-specific function names. As a result, in TFLint v0.51 terraform-specific functions always return unknown values.

This PR fixes the function names so that the terraform-specific functions return the known values.

• https://developer.hashicorp.com/terraform/language/functions/terraform-encode_tfvars
• https://developer.hashicorp.com/terraform/language/functions/terraform-decode_tfvars
• https://developer.hashicorp.com/terraform/language/functions/terraform-encode_expr