hclwrite: handle legacy dot access of numeric indexes

[bendrucker]

This PR allows hclwriter to handle dot syntax for index accesses, e.g.:

module "test" {
  test = kubernetes_config_map.test.metadata.0.name
}

Previously, attempting to partition on an open bracket caused a panic:

https://github.com/hashicorp/hcl/blob/hcl2/hclwrite/parser.go#L408

This PR adds a PartitionTypeOk that is a copy of PartitionType. It returns the same values, plus an ok bool signifying that the token type was found. Otherwise, it returns zero-value inputTokens objects and a false boolean indicating that a partition token could not be found.

The approach to detecting that a traversal is .# style is admittedly fairly naive. However, adding a passing test on a = foo[bar.baz] makes me feel a bit more confident.

Related bug reports:

apparentlymart/terraform-clean-syntax#5
bendrucker/terraform-cloud-migrate#3

I tested a copy of this patch and confirmed that it resolves the second report.

Thank you for all your work here, it has been a joy to develop on! Looking forward to contributing more.

[alisdair]

Thanks for the diagnosis and fix, @bendrucker! 🎉