make JACK backend available cross platform

[Be-ing]

Since version 0.9, the JACK bindings are usable on Windows and macOS, however cpal only uses JACK for cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd")). This will require more than simply changing Cargo.toml because cpal currently assumes that JACK is always available if the feature is enabled.

When this is implemented, I think the JACK backend should be enabled by default.