`registerWorker` typing does not allow async function returning a transferable struct

[mdastous-bentley]

Trying to call registerWorker on a WorkerImplementation that contain an async function returning a transferrable (i.e. {result, transfer}), will give this typing error:

Type '() => Promise<{ result: number; transfer: never[]; }>' is not assignable to type '() => WorkerReturnType<() => Promise<number>>'.
  Type 'Promise<{ result: number; transfer: never[]; }>' is not assignable to type 'WorkerReturnType<() => Promise<number>>'.
    Type 'Promise<{ result: number; transfer: never[]; }>' is not assignable to type 'Promise<number>'.
      Type '{ result: number; transfer: never[]; }' is not assignable to type 'number'.

To Reproduce This PR can be used to reproduce the error with the actual tests.

Expected behavior No typing error.

• OS: Windows
• Browser : chrome
• Version: 129
• iTwin.js Version: 4.9.1