refactor(python): split FSharpRef.__init__ into overloads (#4638)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dbrattli

src/fable-library-py/fable_library/core/types.pyi

@@ -1,7 +1,11 @@
 from collections.abc import Callable
+from typing import overload
 
 class FSharpRef[T]:
-    def __init__(self, contents_or_getter: T | Callable[[], T], setter: Callable[[T], None] | None = None) -> None: ...
+    @overload
+    def __init__(self, contents_or_getter: T) -> None: ...
+    @overload
+    def __init__(self, contents_or_getter: Callable[[], T], setter: Callable[[T], None]) -> None: ...
     @property
     def contents(self) -> T: ...
     @contents.setter