`function/direxists`: Add function that checks if a directory exists

[austinvalle]

Ref: https://github.com/hashicorp/terraform/issues/33394, https://github.com/hashicorp/terraform/issues/25316, https://github.com/hashicorp/terraform/pull/29996

This PR introduces a new provide-defined function for checking the existence of a directory on the filesystem running Terraform. Similar to fileexists.

terraform {
  required_providers {
    local = {
      source = "hashicorp/local"
    }
  }
  required_version = ">= 1.8.0"
}

output "example_output" {
  value = provider::local::direxists("${path.module}/example-directory")
}

 $ tree          
.
├── example-directory
└── function.tf

2 directories, 1 file

 $ terraform plan

Changes to Outputs:
  + example_output = true

You can apply this plan to save these new output values to the Terraform state, without changing any real infrastructure.

Notes

• I matched the implementation to fileexists for consistency, but if we prefer deviating we could make some changes
  • Biggest sticking point seems to be returning an error rather than a false if "something" exists at the path but is not a directory. I'm not sure returning false in that scenario is better, so I kept with consistency.
  • If we do keep the error, we should consider mimicking the terraform core logic of pretty printing the type of file it actually is in the error message: https://github.com/hashicorp/terraform/blob/8da1c006f9d7f381e644a5f0e2c25094ea29571d/internal/lang/funcs/filesystem.go#L234-L257
• I named the files function_ to follow the convention of the repository. I can also refactor all the files in this provider to match the other accepted pattern of:
  • *_function.go
  • *_resource.go
  • *_data_source.go

[bflad]

terraform-plugin-docs is now updated on main 👍

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