constructors: Fix defaulted TypeVars (#7965)

From the list in https://github.com/microsoft/pyright/issues/3501

stdlib/contextvars.pyi

@@ -14,7 +14,10 @@ _P = ParamSpec("_P")
 
 @final
 class ContextVar(Generic[_T]):
-    def __init__(self, name: str, *, default: _T = ...) -> None: ...
+    @overload
+    def __init__(self, name: str) -> None: ...
+    @overload
+    def __init__(self, name: str, *, default: _T) -> None: ...
     @property
     def name(self) -> str: ...
     @overload