Add a new .msix extension

Microsoft was announced a new extension of installer for Windows. I just added that one to be ignored as default, like others installers files.
References:
https://blogs.windows.com/buildingapps/2018/03/07/three-things-need-know-windows-developer-day/
https://github.com/Microsoft/msix-packaging

Global/Windows.gitignore

@@ -15,6 +15,7 @@ $RECYCLE.BIN/
 # Windows Installer files
 *.cab
 *.msi
+*.msix
 *.msm
 *.msp