Formatter: improved_async_statements_handling preview style

[MichaReiser]

Implement Black's same as improved_async_statements_handling as a preview style. This is the same as #8889 but for async context managers. Requires Py39+

async def func() -> (int):
    return 0


@decorated
async def func() -> (int):
    return 0


async for (item) in async_iter:
    pass

Formatted as (already implemented in ruff)

async def func() -> int:
    return 0


@decorated
async def func() -> int:
    return 0


async for item in async_iter:
    pass

and

async def func():
    async with \
        make_context_manager1() as cm1, \
        make_context_manager2() as cm2, \
        make_context_manager3() as cm3, \
        make_context_manager4() as cm4 \
    :
        pass

    async with some_function(
        argument1, argument2, argument3="some_value"
    ) as some_cm, some_other_function(
        argument1, argument2, argument3="some_value"
    ):
        pass

gets formatted as

async def func():
    async with (
        make_context_manager1() as cm1,
        make_context_manager2() as cm2,
        make_context_manager3() as cm3,
        make_context_manager4() as cm4,
    ):
        pass

    async with (
        some_function(argument1, argument2, argument3="some_value") as some_cm,
        some_other_function(argument1, argument2, argument3="some_value"),
    ):
        pass