Follow up of the terraform_map_duplicate_keys rule

[wata727]

Follow up of #194

This fixes some issues with the terraform_map_duplicate_keys rule added in #194.

First, EvaluateExpr call fails due to terraform-linters/tflint-plugin-sdk#338 if map key is a naked identifier.

$ cat main.tf
locals {
  map = {
    foo = 1
    bar = 2
    bar = 3 # duplicate key
  }
}
$ tflint
Failed to check ruleset; failed to check "terraform_map_duplicate_keys" rule: <nil>: failed to evaluate expression; main.tf:3,5-8: Invalid reference; A reference to a resource type must be followed by at least one attribute access, specifying the resource name., and 2 other diagnostic(s)

To prevent this, we need to get the values of the keys in a way similar to ObjectConsKeyExpr.Value as a workaround.
https://github.com/hashicorp/hcl/blob/v2.21.0/hclsyntax/expression.go#L1311-L1338

Second, since val.AsString can panic on sensitive values ​​or non-string keys, This PR made it ignore these.

variable "sensitve"  {
  default = "secret"
  sensitive = true
}

locals {
  map = {
    (var.sensitive) = 1 # sensitive key
    1 = 2               # number is converted to string
    {} = 3              # invalid in the Terraform language, but valid in the HCL layer
  }
}

Finally, I fixed an issue where added rules were not registered in presets and there was a lack of documentation.

[wata727]

@bendrucker Could you take a look at this?
I'd especially like some input on whether this rule should be added to the recommended preset.

[bendrucker]

Thanks! Agree that it makes sense in the recommended preset.