WIP: Add TextInput.setMarkedTextRect handler for Windows

[cbracken]

When inputting text via multi-step text input, such as via Chinese,
Japanese, or Korean IME input methods, the framework computes the
rectangle of the compose text and updates the embedder via the
TextInput.setMarkedTextRect message over the channel.

We add a handler for this method which stores the composing rect as a
field on TextInputPlugin.

In a followup patch, a similar handler will be added for the
TextInput.setEditableSizeAndTransform message. Them when compose rect is
needed in window coordinates, the rect can be multipled by the 4x4
transform matrix, so the result may be fetched by the view/window class
and the IME candidates window position updated.

Issues

This is one in a series of patches addressing flutter/flutter#65574: Full IME support for Windows

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Included in #23795.