Async func exports unimplemented in bindgen

`componentize-js` 0.20.0 panics with `not yet implemented` when a WIT world exports an async function, because the export path in `spidermonkey-embedding-splicer/src/bindgen.rs` hits `todo!()` for all three async function kinds:

https://github.com/bytecodealliance/ComponentizeJS/blob/main/crates/spidermonkey-embedding-splicer/src/bindgen.rs#L485-L487

```rust
FunctionKind::AsyncFreestanding => todo!(),
FunctionKind::AsyncMethod(_id) => todo!(),
FunctionKind::AsyncStatic(_id) => todo!(),
```

This blocks targeting any WIT world whose exports use `async func`, including the component-model-async style used by wasmtime 43's `component-model-async` feature and WASI preview 3 worlds that declare async exports.