I've seen a few tools like this that try to keep things in sync. While it is better than the alternative of doing it manually, it is still a losing battle and not the greatest DX. Often you still have validation (if it exists) and types that also need to be kept in sync, and documentation is scattered throughout the codebase.
Another approach is to turn the example into a schema - and have it be included in the env loading process. This way non-sensitive values can be included directly, and it can never be out of sync. Only values that differ need to be defined in your git-ignored files or injected by the environment. This is precisely how https://varlock.dev solves this problem. The schema becomes the single source of truth, and is used to generate types as well. It also provides validation, leak prevention, and a bunch of other nice tools.
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Other env-syncing libraries - https://github.com/luqmanoop/sync-dotenv - https://www.npmjs.com/package/dotenv-safe - https://www.npmjs.com/package/generate-env-example