Support `TypeExpr` types

```py
def f(t: TypeExpr[T]) -> T: ...

reveal_type(f(int | str))  # int | str

a: TypeExpr[int | str] 
a = int | str  # valid UnionType
a = int  # valid int
a = 1  # valid, bare literal
a = object  # invalid, object isn't a subtype of int | str
```

I can imagine a lot of cases where there is ambiguous stuff like `t: TypeExpr[Any]`, so we can just ban those ones.
