[Python] Generate async def for F# task { } expressions

[dbrattli]

Why

Python frameworks like FastAPI require actual async def functions to properly handle async I/O. Previously, Fable generated regular def functions returning Awaitable[T], which prevented FastAPI from detecting async endpoints.

How

• F# task { } expressions now transpile to Python async def functions instead of regular functions returning Awaitable[T]
• Return type is unwrapped from Task[T] to T for async functions (Python requires this)
• Return statements are automatically wrapped with await