[Win32, Keyboard] Correctly process modifier events in IME mode

[dkwingsmt]

Although the Win32 keyboard logic correctly ignores IME key events from letter keys (identified by virtual key VK_PROCESSKEY), it didn't handle modifier keys during IME mode well, which is a result of two mistakes:

1. It incorrectly synthesizes a key down event, since the key state is set, while the virtual key is not the usual virtual key for the modifier.
2. It incorrectly dispatches a non-synthesized key down event if the extended bit is set, because result_logical_key will not be 0xe5 but with an extended bit.

This PR fixes the first problem by moving the VK_PROCESSKEY branch to before modifier synthesization, and adding an exception rule to the logical key conversion. As a result, IME events will no longer trigger modifier synthesization at all. It's a little sad, but acceptable.

Fixes flutter/flutter#95888.

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[gspencergoog]