feat: allow .env files in project directories to configure environment

[HipsterBrown]

Since one of the complexities of managing the xs-dev environment is making sure certain environment variables are sourced in the correct shell profile, maybe per-project configuration can use a conventional .env file to load into the process before certain commands like xs-dev run.

Along with existing configuration variables like UPLOAD_PORT, there could be a DEPLOY_TARGET or DEPLOY_DEVICE setting for the default --device (or platform target) for running projects.

[dashcraft]

I think a .env file is going to go a long way in improving build configurations with minimal effort. It could be mostly self documenting by naming convention.

I would probably prefer having default settings behind the scenes and then being able to override with a .env, than a .env per project.