Support bare `depends_on :macos` in casks

- Allow casks to use `depends_on :macos` without a version
  to explicitly mark them as macOS-only
- This mirrors the formula behaviour and lets `brew bundle`
  and search filtering detect macOS-only casks via
  `supports_linux?` returning false
- On Linux the bare requirement is unsatisfied, blocking
  install; on macOS any version satisfies it
- Rename `macos_req` to `macos_requirement` per review