feat: Change the delimiters for state templates and release templates

[mumoshu]

As @sebastien-prudhomme suggested in #515:

I've just discovered the edge case for Prometheus AlertManager and was surprised by the fact that helmfile.yaml is a template.

It seems that it's possible to change the curly bracket separator with other separator. Have you considered that option?

After quick thinking I have following options:

• s{ ... } for state templates
• r{ ... }
• Set HELMFILE_EXPERIMENTAL to require .gotmpl file ext.(.gotmpl should be required to enable templating of helmfile.yaml #584) to enable the new delimiters while disabling the old delimiters. Maybe the behavior of HELMFILE_EXPERIMENTAL will be the default in near future. But this allows us to keep backward-compatibility for a while anyway.

Any thoughts?

[mumoshu]

Neither s{ } nor r{ } seem to work in cases like s{ {{.Foo.Bar}} } as the first } seen in Bar}} is considered the closing curly bracket.