feat: Increase security by virtualizing $env/static/*

[tcc-sejohnson]

Previously, the contents of $env/static/* were written to the disk in .svelte-kit/runtime/env/static/*.js. We'd like to prevent dumping the environment to the disk, so this is a test implementation of using virtual modules.

A couple of questions for those wiser than I:

1. How do we make this work with build? It seems to work in dev, but throws a big red error when running build.
2. Are there any cases where this might come back to bite us?

Big thanks to @dominikg for the first "test" implementation of this.

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[tcc-sejohnson]

Extracted the env module replacement out into its own Vite plugin, adding it to the array returned by sveltekit. Tadaa!