Fix async worker return type (backport #7249) [release/4.9.x]

[mergify]

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.

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This is an automatic backport of pull request #7249 done by Mergify.