feature request: support configuration via config file

[simonhaenisch]

Basically to support a ~/.corepackrc file that allows the same config as the COREPACK_ environment variables.

Could be exactly like ~/.npmrc with equivalent npm_config_ env vars, or a yarn-like ~/.corepackrc.yml would also work (I don't care about the format really).

We have a use case for this for automatic machine setup, where adding a config dot-file to the user's home folder is a lot more elegant than modifying their shell's rc file to export extra variables (I generally dislike the idea of automatically modifying user config files).