Conditional types with `If`

Would make a lot of overloads simpler
```py
def foo(a: bool) -> If[a, str, int]:
if a:
return str(a)
return 1

def bar(s: str) -> If[s == "AMONGUS", str, int]:
if a == "AMONGUS":
return str(a)
return 1
```

In the same way that overloads are not checked, these also wouldn't be.

What about `if`?
```py
def foo(a: bool) -> str if a else int: ...
```

No file, test, or entry point is named. Start by tracing how basedmypy handles overload checking and parses function return annotations, then compare the proposed `If` form with the conditional `if` annotation form. Done would require a settled design for both forms and tests defining their supported behavior.

Written by the indexing model from the issue text.