Support GASKET_ENV with process exposure

[agerard-godaddy]

Summary

For situations where an app needs to known the gasket CLI settings without access to the actually GasketEngine instance. Also introduces the distinct GASKET_ENV var, which will still fallback to NODE_ENV if neither it nor --env flag are set.

Changelog

@gasket/cli

• Support for GASKET_ENV with fallback to NODE_ENV
• expose gasket settings on process.env

@gasket/plugin-command

• Support for GASKET_ENV with fallback to NODE_ENV

@gasket/plugin-start

• Support for GASKET_ENV with fallback to NODE_ENV

Test Plan

• Updated unit tests and locally testing in an app