[ty] Dataclasses: __hash__ semantics and unsafe_hash (#21470)

## Summary

Implement the semantics of `__hash__` for dataclasses and add support
for `unsafe_hash`

## Test Plan

New Markdown tests.

crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md

@@ -362,9 +362,71 @@ class AlreadyHasCustomDunderLt:
         return False
 ```
 
-### `unsafe_hash`
+### `__hash__` and `unsafe_hash`
 
-To do
+If `eq` and `frozen` are both `True`, a `__hash__` method is generated by default:
+
+```py
+from dataclasses import dataclass
+
+@dataclass(eq=True, frozen=True)
+class WithHash:
+    x: int
+
+reveal_type(WithHash.__hash__)  # revealed: (self: WithHash) -> int
+```
+
+If `eq` is set to `True` and `frozen` is set to `False`, `__hash__` will be set to `None`, to mark
+is unhashable (because it is mutable):
+
+```py
+from dataclasses import dataclass
+
+@dataclass(eq=True, frozen=False)
+class WithoutHash:
+    x: int
+
+reveal_type(WithoutHash.__hash__)  # revealed: None
+```
+
+If `eq` is set to `False`, `__hash__` will inherit from the parent class (which could be `object`).
+Note that we see a revealed type of `def …` here, because `__hash__` refers to an actual function,
+not a synthetic method like in the first example.
+
+```py
+from dataclasses import dataclass
+from typing import Any
+
+@dataclass(eq=False, frozen=False)
+class InheritHash:
+    x: int
+
+reveal_type(InheritHash.__hash__)  # revealed: def __hash__(self) -> int
+
+class Base:
+    # Type the `self` parameter as `Any` to distinguish it from `object.__hash__`
+    def __hash__(self: Any) -> int:
+        return 42
+
+@dataclass(eq=False, frozen=False)
+class InheritHash(Base):
+    x: int
+
+reveal_type(InheritHash.__hash__)  # revealed: def __hash__(self: Any) -> int
+```
+
+If `unsafe_hash` is set to `True`, a `__hash__` method will be generated even if the dataclass is
+mutable:
+
+```py
+from dataclasses import dataclass
+
+@dataclass(eq=True, frozen=False, unsafe_hash=True)
+class WithUnsafeHash:
+    x: int
+
+reveal_type(WithUnsafeHash.__hash__)  # revealed: (self: WithUnsafeHash) -> int
+```
 
 ### `frozen`