Add template function isFile

[itscaro]

My use case is to check for file existence before using it with readFile, or we might implement must functions instead.

For now as a workaround, I'm using something like if eq "ok" (exec test -f <file> && echo "ok" || echo "ko") to do the job :-)

[mumoshu]

Linking #766

[agorgl]

Would love to see this!

[mumoshu]

LGTM. Thanks as always for your contribution ☺️

[mhienle]

Can you please give an example of how the funtion should be used inside a helmfile.yaml?

[jamie-sweeney]

I also am interested in seeing a simple example for this use-case, as I am encountering hurdles in usage;
if I succeed I will post my simple example.