How to set a Environment Variables for Frontend

You should not be putting any of those values into your frontend at all. Everything in the frontend files is public, so you must NEVER store private data, such as OAuth tokens or app/extension secrets, in the frontend or the extension config service.

Not only is it in violation of the Twitch Developer Agreement to expose that data to users, but a malicious user could take advantage of those credentials to render your extension unusable.