feat: add rawPublicVars for loadEnv

[chenjiahan]

Summary

Add a new rawPublicVars return value for the loadEnv JS API:

• Can be used to define the process.env object, see [Bug]: can't destructure process.env #3718
• Align the return value of Vite's loadEnv method, see https://vite.dev/guide/api-javascript.html#loadenv

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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