AppVersion treated as decimal number

Hey, Thanks for the awesome tool - one issue that I wanted to raise as didn't see anything about it in the repo is that appVersion sometimes gets treated as a decimal number instead of string due to not being quoted.

This requires specific circumstances - e.g. in our case using commit id as appVersion and bad luck - e.g this is the commit that started breaking our index file `02824e16` but I guess this could be worked around by quoting the appVersion field automatically.

I've checked all of our releases and they all have appVersion quoted in Chart.yaml therefore chart-releaser must be dropping it when copying the info.

Not able to find where this gets created in the code but I found the same issue on the upstream helm3 repo as an example : https://github.com/helm/helm/issues/6849