feat: expose function for validation without running Vite

[aweebit]

Fixes #34

[Julien-R44]

Thanks for the PR ! Just did some refactor

Also, no need to use Object.assign(process.env, env) -> loadAndValidateEnv will be responsible for it, but still returns the env mapping if need

[aweebit]

@Julien-R44 Big thanks! I've now replaced the old bulky workaround in my project with a loadAndValidateEnv call, and it works like a charm 🥳