Is your feature request related to a problem? Please describe.

Users are accustomed to being able to set individual Zarf variables or Helm values directly. We should have parity, probably with a --set-value key that can be passed in multiple times, or with comma separated values.

Another thing to consider is that we'll eventually want the --set flag to provide this functionality for Values. Right now, --set sets Variables, so as a part of implementing this, we should add --set-variable and alias the existing --set flag to this. Eventually we'll swap from one to the other, either at Values GA or sometime after.