add underscore to HCL function identifier regex

[keeakita]

As titled. While I realize that this is similar to #185 which was closed, I believe that just adding the underscore will keep the regex idiomatic while covering common, real-world cases.

The HCL spec here:
https://github.com/hashicorp/hcl/blob/main/hclsyntax/spec.md#identifiers

contains the following paragraph:

The dash character - is additionally allowed in identifiers, even though
that is not part of the unicode ID_Continue definition. This is to allow
attribute names and block type names to contain dashes, although underscores
as word separators are considered the idiomatic usage.

I believe this sufficiently implies that the use of _ as an idiomatic word-separator character in identifiers.

Additionally, we can see real-world adoption of this practice in projects like Terragrunt, which use the underscore as a word separator for built-in functions: https://terragrunt.gruntwork.io/docs/reference/built-in-functions/