Set environment variable when running `svelte-kit package`

Describe the problem

When writing a preprocessor, I've found it desirable to do different things (e.g warning vs error, optimize vs deoptimize etc.) based on the mode (e.g prod, dev etc.) that is running. Svelte-kit sets NODE_ENV to "development" and "production" for the dev and build modes respectively. It would be helpful if Svelte-kit does the same for svelte-kit package to allow different tools to behave accordingly.

Describe the proposed solution

Before starting packaging simply set the process NODE_ENV to "packaging".

Alternatives considered

Require that consumers of your preprocessor/build-tool MUST set the env variable to get correct behavior during packaging:

NODE_ENV=packaging npx svelte-kit package

This is less convenient especially given that Svelte-kit "knows" that it's running in packaging mode.

Importance

nice to have

Additional Information

NODE_ENV is currently undefined during packaging.

Locate the entry point for the svelte-kit package command and trace where packaging begins. Set NODE_ENV to packaging before that work starts, then verify that the environment variable is available during packaging without changing the existing dev or build behavior.