[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/src/semantic_index/builder.rs

@@ -2805,7 +2805,9 @@ impl<'ast> Unpackable<'ast> {
         match self {
             Unpackable::Assign(_) => UnpackKind::Assign,
             Unpackable::For(_) | Unpackable::Comprehension { .. } => UnpackKind::Iterable,
-            Unpackable::WithItem { .. } => UnpackKind::ContextManager,
+            Unpackable::WithItem { is_async, .. } => UnpackKind::ContextManager {
+                is_async: *is_async,
+            },
         }
     }