Infer descriptor type

This is a copy of this enhancement I proposed for mypy: https://github.com/python/mypy/issues/10246

I use this inference in a project through a mypy plugin, I'm trying out pyrefly so I'm curious if this is something you'd consider allowing or not (feel free to close the issue in that case).

## Feature

For the common case of a descriptor which behaves like a normal attribute, infer:

```
class X:
y: int = descriptor()
```

to:

```
class X:
y: descriptor[int] = descriptor()
```

if descriptor is a Generic with one parameter. Extra checks should also be done to make sure that the descriptor is properly typed to behave like an attribute of that type (`__get__` and `__set__` expect that type on an instance).

## Pitch

Type annotations apply to object instances by default, so it's strange to see y: descriptor[int] as type annotation while x.y returns an int.
You could also see a normal attribute as an invisible descriptor whose behavior is to set __dict__[name] (and raise AttributeError on the class), so this could be a way to unify these things.

## Other

This is kind of how dataclasses are typed (although dataclasses have more magic involved).

Possibly this could be opt-in through some mechanism like a metaclass or a class decorator, to make it more explicit.

[sandbox example](https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEALqcROgOZ0Q3G6UN0AqADrow1eplQNUAYyio4ceJ269%2Bk6XIVwANHUgwomESID6AFQCaABQCidALx1zzGADVUlABRCQFm7a%2BAJQm6CJainQAwrjocNLoDF4A4jDoMJQQMgDa-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-tjw3d9PxnV453DWAy00PNfSCVcQKnL9Zz3KDywiOAjyHOgAEEGAYTJsEVBgYFsShqEoEQQB0EBiOgTCyAodcAFUGGgepSH0RV0BkNiOlCZEwF4GgpFMdBFRobAMi8atEg5Oh4koBNNW1fRfBuCSpKUuhgHwABfXwqJosgFTAKBSEIBhaCgJi6GsUhTPMhSMBwAg6HGSA2GVKQIFiQgRHXABlGAH1jfDiDgRAAHoopMypzMIXg2CitIoswXAZDgKKPIgLzKB82Iov0Xg6FQZZUGgVBsFgdzYk87y%2BO2XBiEazCRDIBhY1iD5YTgXzticXwAGZCB9AAmQz0BAPSaNkNjVgAMWgGAKDQLA8CIRi9KAA)