UI: split screenshot area into a separate operator

While the screenshot operator showed an "Area" option,
it wasn't usable from the main menu (unless accessed via menu search).

Split screenshot area into an operator that depends on cursor.

release/scripts/startup/bl_ui/space_topbar.py

@@ -655,6 +655,7 @@ class TOPBAR_MT_window(Menu):
         layout.separator()
 
         layout.operator("screen.screenshot")
+        layout.operator("screen.screenshot_area")
 
         if sys.platform[:3] == "win":
             layout.separator()