A `Unionize` typevar and annotaiton modifier

```py
T_un = TypeVar("T_un", unionize=True)

def test1(a: T_un, b: T_un): ...
test1("", 1) # T is str | int

class A: ...
class B(A): ...
class C(A): ...

def test2(a: Unionize[T], b: T, c: T): ...
test2("", B, C) # T is str | A

# maybe even this
condition: bool
a: Unionize = 1 if condition else "" # int | str
```

Start with the proposed TypeVar(unionize=True), Unionize[T], and bare Unionize examples in the issue. Define the intended type-inference and annotation semantics, including whether the final example is in scope; done requires an agreed behavior and tests for the shown cases.

Written by the indexing model from the issue text.