Add augmented assignment inference for -= operator (#13981)

## Summary

See: https://github.com/astral-sh/ruff/issues/12699

crates/red_knot_python_semantic/resources/mdtest/assignment/augmented.md

@@ -0,0 +1,35 @@
+# Augmented assignment
+
+## Basic
+
+```py
+x = 3
+x -= 1
+reveal_type(x)  # revealed: Literal[2]
+```
+
+## Dunder methods
+
+```py
+class C:
+    def __isub__(self, other: int) -> str:
+        return "Hello, world!"
+
+x = C()
+x -= 1
+reveal_type(x)  # revealed: str
+```
+
+## Unsupported types
+
+```py
+class C:
+    def __isub__(self, other: str) -> int:
+        return 42
+
+x = C()
+x -= 1
+
+# TODO: should error, once operand type check is implemented
+reveal_type(x)  # revealed: int
+```