Add ability to set a default target environment

[mikkelhegn]

With the new target-environment feature, it would be helpful to have a way to configure your Spin CLI installation to always default to a defined target-environment, so that every spin new wouldn't need to be supplied the -E or --target-environment flag if you usually build for just one target platform.

I'm not sure if there's a config feature in Spin at this point but e.g., spin config default-target-environment=my_environment would be along the lines. Or less sophisticated through an environment variable.