support inlined process.env.DEBUG

[timvandam]

Some browser-like environments do not support localStorage nor process.env, but can be provided environment variables because they automatically get inlined (i.e. process.env.XYZ gets replaced by whatever value XYZ is during a build step). This change supports this scenario without breaking previous behavior.

As additional context, I am running into issues when trying to use debug in a Shopify POS extension. These extensions run in a sandboxed version of JS that lacks many built-ins like localStorage. Shopify's build step automatically inlines environment variables, but this still failed because process would not be defined, which the previous code required