fix: increment version in Chart.yaml based on nextRelease.version

[nflaig]

Currently, the version in Chart.yaml is manually increment by using semver package which is not the common approach used in other plugins, e.g. npm plugin. This means that currently there is a bug that pre-releases are not supported because semver.inc does not consider that.

Regarding the appVersion value in Chart.yaml I am not really sure what to do with it but I guess if it is a 1-to-1 relation between app version and chart version it should be the same. But in 99% of the charts that exist the appVersion is independent from the chart version and usually updated manually when the default image tag referenced in the values file is updated or the appVersion value is directly used as the image tag in some cases.

Closes #20