[ty] Add support for properties that return Self (#21335)

## Summary

Detect usages of implicit `self` in property getters, which allows us to
treat their signature as being generic.

closes https://github.com/astral-sh/ty/issues/1502

## Typing conformance

Two new type assertions that are succeeding.

## Ecosystem results

Mostly look good. There are a few new false positives related to a bug
with constrained typevars that is unrelated to the work here. I reported
this as https://github.com/astral-sh/ty/issues/1503.

## Test Plan

Added regression tests.

crates/ty_python_semantic/resources/mdtest/annotations/self.md

@@ -139,7 +139,7 @@ The first parameter of instance methods always has type `Self`, if it is not exp
 The name `self` is not special in any way.
 
 ```py
-def some_decorator(f: Callable) -> Callable:
+def some_decorator[**P, R](f: Callable[P, R]) -> Callable[P, R]:
     return f
 
 class B:
@@ -188,10 +188,10 @@ class B:
 reveal_type(B().name_does_not_matter())  # revealed: B
 reveal_type(B().positional_only(1))  # revealed: B
 reveal_type(B().keyword_only(x=1))  # revealed: B
+# TODO: This should deally be `B`
 reveal_type(B().decorated_method())  # revealed: Unknown
 
-# TODO: this should be B
-reveal_type(B().a_property)  # revealed: Unknown
+reveal_type(B().a_property)  # revealed: B
 
 async def _():
     reveal_type(await B().async_method())  # revealed: B

crates/ty_python_semantic/resources/mdtest/properties.md

@@ -49,6 +49,40 @@ c.my_property = 2
 c.my_property = "a"
 ```
 
+## Properties returning `Self`
+
+A property that returns `Self` refers to an instance of the class:
+
+```py
+from typing_extensions import Self
+
+class Path:
+    @property
+    def parent(self) -> Self:
+        raise NotImplementedError
+
+reveal_type(Path().parent)  # revealed: Path
+```
+
+This also works when a setter is defined:
+
+```py
+class Node:
+    @property
+    def parent(self) -> Self:
+        raise NotImplementedError
+
+    @parent.setter
+    def parent(self, value: Self) -> None:
+        pass
+
+root = Node()
+child = Node()
+child.parent = root
+
+reveal_type(child.parent)  # revealed: Node
+```
+
 ## `property.getter`
 
 `property.getter` can be used to overwrite the getter method of a property. This does not overwrite