Fix async functions generating synchronous trampolines (boxed#454)

[DamienGR]

This commit fixes the issue where mutmut 3.4.0 generates synchronous wrapper trampolines for async functions, which breaks frameworks like FastAPI that use asyncio.iscoroutinefunction() to inspect function signatures.

Changes:

• Added is_async and is_async_generator parameters to build_trampoline()
• Created _mutmut_trampoline_async() for async functions that properly awaits the original/mutant functions
• Async generators now use a wrapper that yields from the original async generator to preserve async iteration behavior
• Added _contains_yield() helper to detect generator functions
• Updated tests to handle whitespace changes in generated code

The fix ensures:

• asyncio.iscoroutinefunction() returns True for async function wrappers
• Async generators work correctly with 'async for' iteration
• Regular sync functions continue to work as before

Fixes boxed#454