* 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
f8dfe2fab4