Deprecate JsStatic in favor of #[wasm_bindgen(thread_local)]

[daxpedda]

This PR deprecated JsStatic in favor of #[wasm_bindgen(thread_local)], which creates a std::thread::LocalKey. The syntax is otherwise the same.

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(thread_local)]
    static FOO: JsValue;
}

JsStatic was pretty much a hack that exposed a thread local variable by transmuting the lifetime to 'static, which does make sense on Web, but not with multi-threading, which was making this rather unsound.

Additionally, impl Deref for JsStatic is not present anymore when compiling with cfg(target_feature = "atomics") to remove the possibility of unsoundness entirely.