Support for .env files

[jfallows]

Describe the desired outcome from the user's perspective
As devops, I want to be able to source control some environment-based configuration for zilla.yaml.

Acceptance criteria

• Check for presence of .env file in current directory where zilla start is executed, and use to help resolve ${env.VAR}
• Allows name of .env file to be overridden by command line argument zilla start --env-file <FILENAME>

Additional context
See https://nodejs.org/dist/latest-v20.x/docs/api/cli.html#--env-fileconfig for details on priority for environment variable resolution.