Added specs for SplitMenuFlyoutItem control #10862
                
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This is an API Spec PR for the proposed SplitMenuFlyoutItem control.
PR Type
Please check the type of change your PR introduces:
Description
The SplitMenuFlyoutItem control is proposed to be a new addition to the WinUI library, designed to provide a split button experience within a menu flyout. This control derives from MenuFlyoutItem and introduces a dual-button interface consisting of a primary button and a flyout button.
Motivation and Context
This functionality will allow developers to expose a default primary action while also offering additional options through an attached dropdown — ideal for condensing complex functionality into a smaller footprint and saving overall menu length.
How Has This Been Tested?
Screenshots (if appropriate):
Attached in the document.