feat: async support

I've tried to use async functions, however it fails to compile with the error that splicer_js_api is not available.
I didn't found any implementation for [splicer_js_api](https://github.com/laptou/jsi-rs/blob/c90f7d4a83b0545f38a884fe8726f8d7c7cf8cba/jsi-macros/src/host_object.rs#L392C29-L392C43) in the codebase nor in crates.io.
What is the current state of async support?