Support string annotations in functional class annotations

E.g., Mypy and Pyright both support this:
```python
from typing import NamedTuple

NT = NamedTuple("NT", [("field", "set[int]")])

NT(1)  # error: Argument 1 to "NT" has incompatible type "int"; expected "set[int]"  [arg-type]
```

Today, ty errors on `"set[int]"` with:

```
Object of type `Literal["set[int]"]` is not valid as a `NamedTuple` field type(invalid-type-form)
```