Add loadEnvFile

[m-rinaldi]

Description of the change

This PR adds bindings for process.loadEnvFile(path), available since Node v20.12.0.

• loadEnvFile :: Effect Unit loads .env from the current working directory
• loadEnvFile' :: String -> Effect Unit loads the given file

As in Node, existing variables keep their value. A missing file throws. The pair follows exit/exit' style.

---

Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)