feat(core): support validating environment variables

[innocenzi]

This pull request adds validation support to environment variables:

env('SLACK_TOKEN', rules: [new IsNotNull(), new StartsWith('xoxb')]);

This allows userland code and third-party packages to be stricter about their expectations and avoid unexpected errors or behavior discrepancy down the line, particularly when changing environments (eg. forgetting to add an environment variable in CI or in production).