Template function namespaces

I would like to provide a comprehensive suite of functions to my templates. Sadly the only separator character available for function names (Identifier) is "_".

Given the following template:
```hcl
// Ideal
${config:fetch("keyname", "default")}
// or
${config::fetch("keyname", "default")}
// Less ideal
${config.fetch("keyname", "default")}
```
Is there any way I can accomplish such a thing?

Start by reviewing the HCL template expression parser and the rules for function-name identifiers; the issue does not name a file or test entry point. Compare the requested colon, double-colon, and dot namespace forms, then establish the accepted syntax and corresponding test coverage before implementation.

Written by the indexing model from the issue text.