Added a WasmerEnv abstraction to the CLI

[Michael-F-Bryan]

This adds a WasmerEnv struct which abstracts over $WASMER_DIR and config used to communicate with the registry (i.e. graphql endpoint and tokens).

Now, whenever you run a command that interacts with the registry in some way, you should always be able to use the $WASMER_DIR, $WASMER_REGISTRY, and $WASMER_TOKEN environment variables or their associated command-line arguments.

This makes the handling of things like $WASMER_DIR, --token, and --registry consistent across all our subcommands.

Fixes #3993, fixes #3959.

[syrusakbary]

This PR made me super confused.

I think this PR is more complicated than it should. It also ties the WASMER_TOKEN into a Wasmer dir? I don't think this make much sense, what am I missing?

[Michael-F-Bryan]

WasmerDir is a bad name so I've renamed it to WasmerEnv. The $WASMER_TOKEN is actually pretty coupled to $WASMER_DIR and $WASMER_REGISTRY because we want to go through the global config when the $WASMER_TOKEN environment variable isn't set.

It may feel overcomplicated, but we want to have a proper abstraction for working with the $WASMER_DIR and tokens and all that. We currently have about 10 different versions of --token or --registry, all with slightly different behaviours, fallbacks, sometimes one command will respect environment variables while another command won't, etc. This PR is fixing those inconsistencies by extracting the logic into its own component.

[theduke]

Love it.

The Edge CLI followed a very similar approach.

Left a few comments/discussion points.