Fix async worker return type

[pmconne]

Fixes #7248.

7088 enabled support for asynchronous implementations of individual worker operations, to prevent long-running operations from blocking subsequent requests. But, the types did not accurately capture these semantics, so you couldn't return { result: T, transfer: Transferable[] } from async implementations without the compiler complaining. It would expect you to return { result: Promise<T>, transfer: Transferable[] } instead.

Adjusted the types to fix this.

[mdastous-bentley]

@aruniverse I would like to get it backported to 4.9.x

[aruniverse]

@mergifyio backport release/4.9.x

[mergify[bot]]

backport release/4.9.x

✅ Backports have been created

* [#7250 Fix async worker return type (backport #7249) [release/4.9.x]](https://github.com/iTwin/itwinjs-core/pull/7250) has been created for branch `release/4.9.x`