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* Win32 a11y bridge and platform node delegates This is the third in a series of patches adding accessibility support for the Windows embedder. This patch wires in the Accessibility bridge, and lands the core structure of the Windows FlutterPlatformNodeDelegate and AccessibilityBridgeDelegate classes, including: * Instantiating the AccessibilityBridge when the semantics tree is enabled. * Creating FlutterPlatformNodeDelegate wrappers for semantics tree nodes. * Handling custom action updates. * Building and updating the accessibility tree on semantics updates. * Returning the native IAccessible objects when queried. Breaking this out so that the follow-up patches can be reviewed and landed in smaller, independent chunks. Issue: https://github.com/flutter/flutter/issues/77838 Issue: https://github.com/flutter/flutter/issues/93928