Support helmfile template functions

[DanielHabenicht]

Helmfile value files use template functions beyond Helm/Sprig (env, requiredEnv, exec, readFile, fetchSecretValue, ...), which were reported as "function ... not defined" by validateTemplateSyntax.

Add helmfileFuncMap() registering stubs for those functions on top of helmFuncMap(), and a helmfile validation mode enabled automatically for .gotmpl files or explicitly via --helmfile (needed for stdin).

Fixes #16.

[DanielHabenicht]

I don't know whether the extra flags are useful nor needed. If we go with the one tool approach, we could also just add the functions to the map and skip the extra arguments/flags

[digitalstudium]

I don't know whether the extra flags are useful nor needed. If we go with the one tool approach, we could also just add the functions to the map and skip the extra arguments/flags

Let's skip the extra arguments please

[DanielHabenichtWork]

Let's skip the extra arguments please

updated

[digitalstudium]

What AI review added here:

1. kustomizeBuild is not a Helmfile template function. While Helmfile does have Kustomize integration (e.g., via hooks or the kustomize binary), it is not available as a Go template function inside .gotmpl files. This appears to be a hallucination.

2. The author missed a crucial function that Helmfile explicitly exposes. According to the official Helmfile documentation, Helmfile overrides Sprig's standard get function but makes the original Sprig version available under the alias sprigGet.
   The Bug: Because sprigGet is not added to the helmfileExtras list, helmfmt will fail syntax validation with "function sprigGet not defined" if a user attempts to use it in their Helmfile templates

[DanielHabenicht]

Yes, it is right. I let it cross check it and found another two functions which are documented but where not listed. Next commit should fix it.