chore(Session.new_window): Return window_index functionality

This reverts commit b46e7fa.

Author: tony

src/libtmux/session.py

@@ -635,6 +635,10 @@ def new_window(
         if window_name is not None and isinstance(window_name, str):
             window_args += ("-n", window_name)
 
+        if window_index is not None:
+            # empty string for window_index will use the first one available
+            window_args += (f"-t{self.session_id}:{window_index}",)
+
         if environment:
             if has_gte_version("3.0"):
                 for k, v in environment.items():