[ty] Support async/await, async with and yield from (#19595)

## Summary

- Add support for the return types of `async` functions
- Add type inference for `await` expressions
- Add support for `async with` / async context managers
- Add support for `yield from` expressions

This PR is generally lacking proper error handling in some cases (e.g.
illegal `__await__` attributes). I'm planning to work on this in a
follow-up.

part of https://github.com/astral-sh/ty/issues/151

closes https://github.com/astral-sh/ty/issues/736

## Ecosystem

There are a lot of true positives on `prefect` which look similar to:
```diff
prefect (https://github.com/PrefectHQ/prefect)
+ src/integrations/prefect-aws/tests/workers/test_ecs_worker.py:406:12: error[unresolved-attribute] Type `str` has no attribute `status_code`
```

This is due to a wrong return type annotation
[here](e926b8c4c1/src/integrations/prefect-aws/tests/workers/test_ecs_worker.py (L355-L391)).

```diff
mitmproxy (https://github.com/mitmproxy/mitmproxy)
+ test/mitmproxy/addons/test_clientplayback.py:18:1: error[invalid-argument-type] Argument to function `asynccontextmanager` is incorrect: Expected `(...) -> AsyncIterator[Unknown]`, found `def tcp_server(handle_conn, **server_args) -> Unknown | tuple[str, int]`
```


[This](a4d794c59a/test/mitmproxy/addons/test_clientplayback.py (L18-L19))
is a true positive. That function should return
`AsyncIterator[Address]`, not `Address`.

I looked through almost all of the other new diagnostics and they all
look like known problems or true positives.

## Typing conformance

The typing conformance diff looks good.

## Test Plan

New Markdown tests

crates/ty_python_semantic/resources/mdtest/with/async.md

@@ -17,5 +17,80 @@ class Manager:
 
 async def test():
     async with Manager() as f:
-        reveal_type(f)  # revealed: @Todo(async `with` statement)
+        reveal_type(f)  # revealed: Target
+```
+
+## Multiple targets
+
+```py
+class Manager:
+    async def __aenter__(self) -> tuple[int, str]:
+        return 42, "hello"
+
+    async def __aexit__(self, exc_type, exc_value, traceback): ...
+
+async def test():
+    async with Manager() as (x, y):
+        reveal_type(x)  # revealed: int
+        reveal_type(y)  # revealed: str
+```
+
+## `@asynccontextmanager`
+
+```py
+from contextlib import asynccontextmanager
+from typing import AsyncGenerator
+
+class Session: ...
+
+@asynccontextmanager
+async def connect() -> AsyncGenerator[Session]:
+    yield Session()
+
+# TODO: this should be `() -> _AsyncGeneratorContextManager[Session, None]`
+reveal_type(connect)  # revealed: (...) -> _AsyncGeneratorContextManager[Unknown, None]
+
+async def main():
+    async with connect() as session:
+        # TODO: should be `Session`
+        reveal_type(session)  # revealed: Unknown
+```
+
+## `asyncio.timeout`
+
+```toml
+[environment]
+python-version = "3.11"
+```
+
+```py
+import asyncio
+
+async def long_running_task():
+    await asyncio.sleep(5)
+
+async def main():
+    async with asyncio.timeout(1):
+        await long_running_task()
+```
+
+## `asyncio.TaskGroup`
+
+```toml
+[environment]
+python-version = "3.11"
+```
+
+```py
+import asyncio
+
+async def long_running_task():
+    await asyncio.sleep(5)
+
+async def main():
+    async with asyncio.TaskGroup() as tg:
+        # TODO: should be `TaskGroup`
+        reveal_type(tg)  # revealed: Unknown
+
+        tg.create_task(long_running_task())
 ```