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

### 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.