FEATURE: Ability to manage repository variables; much like it currently does secrets

[actuarysailor]

Is your feature request related to a problem? Please describe.
Currently, this action manages repository secrets, but it does not do variables.

Describe the solution you'd like
Expand scope to handle repository variables.

Describe alternatives you've considered
Use of other tools / actions; but better to have it all in one centralized and more maintainable