Validate asdf environment variables when `bin/asdf` is invoked

[Stratus3D]

Is your feature request related to a problem? Please describe

We've had issues with people setting asdf variables to the wrong values.

See https://github.com/asdf-vm/asdf/issues/1308

Describe the proposed solution

We should validate asdf environment variables and parameters in .asdfrc when bin/asdf is invoked prior to doing anything else. If a parameter is invalid we should exit with a specific error status code and print an error to STDERR explaining the issue so the user can correct it. We should not permit asdf to operate with known invalid config.

Describe similar asdf features and why they are not sufficient

Configuration page in the documentation is clear and fairly concise (https://asdf-vm.com/manage/configuration.html). However, it seems users still make mistakes from time to time. Having configuration validated automatically makes things more fool proof.

Describe other workarounds you've considered

We've had docs for a while and we still get issues on the issue tracker related to bad config.

[hyperupcall]

@Stratus3D Wondering if you're still interested in doing this? I was thinking about doing this recently.