Simplify async iterator types

[tpgusgh]

Fixes #xxxx

Summary

Refactored FileSystemDirectoryHandle and ReadableStream async iterator definitions to:

• Remove undefined or redundant types (AsyncIteratorObject, BuiltinIteratorReturn)
• Use a shared AsyncIterableIterator<T> utility type for clarity and DRY compliance
• Add missing ReadableStreamIteratorOptions interface
• Improve readability and maintainability through consistent structure and comments

These changes ensure more consistent typings and better compatibility with existing TypeScript async iteration patterns.

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[Renegade334]

Generated files

The DOM files ending in .generated.d.ts aren't meant to be edited by hand.

If you need to make changes to such files, make a change to the input files for our library generator.

(This proposal goes against the direction of travel – see microsoft/TypeScript-DOM-lib-generator#1713 for context.)