[ty] Support `**` unpacking of `TypedDict` in dict-literal assignments (#24703)
## Summary
We already support `**` unpacking of `TypedDict` in named `TypedDict`
constructors; this PR extends it to literals annotated as `TypedDict`,
as in:
```python
from typing import TypedDict
class MyTypedDict1(TypedDict):
aaa: int
bbb: int
class MyTypedDict2(TypedDict):
aaa: int
bbb: int
ccc: int
d1: MyTypedDict1 = {
"aaa": 1,
"bbb": 2,
}
d2: MyTypedDict2 = {
**d1,
"ccc": 3,
}
```
Closes https://github.com/astral-sh/ty/issues/1493