`forEachChild` is hard to use with async API

@andrewbranch 已经在做这个了。

开始于 2026年9月1日。

If you try to use the async API, it's a challenge today. The API provides a synchronous tree traversal function (`forEachChild`), but pulling on semantic information is asynchronous.

The problem isn't the mixture itself - it's that `forEachChild` stops walking when the visitor function returns a truthy result. This makes it impractical to use with async APIs because every async function returns a `Promise` which is truthy.

The current workarounds are a little awkward, basically introducing a layer of indirection between the traversal function and `forEachChild` that uses intermediate queues.

I'm not sure if there's an ideal solution - there's at least two problems right now:

- it feels like we could benefit from a `forEachChildAsync` or something like that.
- using `forEachChild` is a foot-gun if you've decided to use the async API.