Refactor user management workflow

Added

• Support for enforcing existing membership in a specified GitHub organization (see checkUser)
• Support for granting organization users admin permissions on the repository they wish to fork (see promoteUser)

Changed

• Updated Jest unit tests to validate all env inputs
• Downgraded all Jest packages from v29 to v28 to resolve mismatch with ts-jest
• Updated README to document new inputs (checkuser, promoteUser) and remove deprecated ones (addUser)

Removed

• Support for sending GitHub organization invitations to users who were not already a member (see addUser)