Seal CommandExt

[taiki-e]

Both unix::CommandExt and windows::CommandExt 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: smol-rs/async-fs#10