HCL autoformat doesn't allow commenting hash values of a list

I have some HCL (specifically, Terraform code) which looks something like this:

```
variable "test" {
default = [
# This is a comment for this list element.
{
key = "value"
},
]
}
```

Running `terraform fmt` changes the formatting of this code to this:

```
variable "test" {
default = [
{
# This is a comment for this list element.
key = "value"
},
]
}
```

This doesn't seem desirable to me because the comment is meant to refer to the entire hash element.

Start with the formatter entry point used by `terraform fmt` and locate tests covering comments on list elements. Reproduce the example from the issue, then update the behavior so a comment for a list element remains attached to that element and add or adjust a regression test.

Written by the indexing model from the issue text.