Correctly convert surrogate UTF-16 to rune on MS-Windows

[kanryu]

Hi,

I was looking for a window client library for Windows/Mac/Linux written in Go that people around the world can use with confidence, that is, can accurately input multiple languages with IME, and I found your project.

Your library doesn't support IME, but I plan to implement it.
First, as a simple fix, I implemented a fix that allows MS-Windows to accurately transfer characters that occur in UTF-16 surrogate pairs to the application.

CursorMoved: physicalX=1280 physicalY=750
KeyboardInput: state=Pressed scanCode=28 virtualKeyCode=e5
ReceivedCharacter: U+29E3D '𩸽'
KeyboardInput: state=Released scanCode=28 virtualKeyCode=Return