Make yew::platform it's own indepenedent crate

[ranile]

yew::platform is a pseudo-runtime that - at the moment - uses tokio or wasm_bindgen_futures based on the feature flags configured. Since the introduction of #2811, it also contains features that are - while used in Yew - quite unrelated to it.

It should become it's own independent crate.

Also see: #2778