fix: Don't enable the zbus/tokio feature on Unix

[DataTriny]

Fixes #515

For simplicity, I chose to also drop the path to spawn a task using the unstable API. We'd have to enable the tokio_unstable flag ourself (previously done by zbus), just to name the task. Yes this would be useful in diagnozing a crash, but we don't test this enough for me to feel confident keeping this unstable code around.

Tested by running the winit example with both runtimes.

[mwcampbell]

@lunixbochs Can you easily test this solution and make sure it does what you need? Thanks.

[mwcampbell]

Given that accesskit_unix doesn't offer an async API at all, but always does its async work on another thread, and that it's now always using zbus's async-io feature, do we even need to offer async-io and tokio feature, or should we just always use async-io? @lunixbochs What do you think?

[DataTriny]

Even when the zbus/async-io feature is enabled, we still spin a tokio executor if the tokio feature is enabled since we don't use zbus's blocking API. It's just that we can't build zbus without picking an async runtime. So I think it's just that tokio users will depend on some async-std dependencies and will have to explicitly depend on accesskit_unix to disable default features.

But I agree that dropping support for tokio entirely would make things simpler though.