[ty] Synthesize NamedTuple `__match_args__` (#25934)
## Summary
Python 3.10 generates `__match_args__` on named tuples so their fields
can be used in positional class patterns. We already synthesize other
generated named-tuple attributes, but did not expose `__match_args__`.
```python
from typing import NamedTuple
class Point(NamedTuple):
x: int
y: str
reveal_type(Point.__match_args__) # tuple[Literal["x"], Literal["y"]]
```
The gradual fallback for named tuples with unknown fields comes from
`NamedTupleFallback`, as proposed in
https://github.com/python/typeshed/pull/15906. (I've included a
temporary copy of that typeshed change until approved.)
