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.

No files, tests, or entry points are named. Start by locating the type-checker code that handles annotations and union/type expressions, then define and test the shown TypeExpr cases, including the rejected object assignment and the ambiguous Any cases.

Written by the indexing model from the issue text.