More type safety for env function key parameter

[ZilvinasAbr]

Thank you for introducing such nice library to solve a key issue of Next.js!
Wanted to share some random idea that could maybe be used for this library.

Currently the implementation of env function checks if provided environment variable key parameter is with NEXT_PUBLIC_ prefix on runtime (if client-side):

 if (isBrowser()) {
    if (!key.startsWith('NEXT_PUBLIC_')) {
      throw new Error(
        `Environment variable '${key}' is not public and cannot be accessed in the browser.`,
      );
    }

    return window[PUBLIC_ENV_KEY][key];
  }

What if instead we achieved this safety statically with Typescript? Example:

// Allow for key to be only of string with prefix `NEXT_PUBLIC_`
export function env(key: `NEXT_PUBLIC_${string}`): string | undefined {
  if (isBrowser()) {
    // No more checks if key has `NEXT_PUBLIC_` necessary
    return window[PUBLIC_ENV_KEY][key];
  }

  noStore();

  return process.env[key];
}

After this change, Typescript would yell if trying to pass a string without such prefix.

Issues with this approach:

• It would be a breaking change for when trying to use env in server-side without the NEXT_PUBLIC_ prefix, as this is currently allowed and just returns the env variable from process.env. But maybe this would be okay since for server-side env variables the user can just take it from process.env directly, or use some other abstraction, like t3-env ?
• It would get complicated if the key was dynamic. You couldn't just pass a string to it. You would need to do some shenanigans in order to pass it, e.g. env(value as NEXT_PUBLIC_${string}).