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.

從 spidermonkey-embedding-splicer/src/bindgen.rs 中 485-487 行附近的 FunctionKind match 開始，接著追蹤周圍的匯出路徑，以及它對同步函式的處理方式。完成的標準是，async 的獨立函式、方法和靜態匯出不再命中 todo!()，並且可以處理一個匯出 async 的 WIT world，而不會出現報告中的 panic。

由索引模型根據 Issue 內容生成。