Seal DirBuilderExt, DirEntryExt, and OpenOptionsExt

[taiki-e]

unix::DirBuilderExt, unix::DirEntryExt, unix::OpenOptionsExt, and windows::OpenOptionsExt are not intended to be implemented by users, and they should be sealed.

Note that this is a breaking change. (However, given the standard library added a new method in the minor version to os-specific extension traits (not sealed) (1, 2), we may not consider this is a breaking change that requires a major version bump.)

Related: https://github.com/smol-rs/async-process/issues/6, https://github.com/smol-rs/async-fs/commit/f4cc6e5e91df5cc8955c63d19775b7670e1cd7a4#commitcomment-42948266