add userprefs for VS for Mac

Author: TylerLeonhardt

.gitignore

@@ -61,4 +61,7 @@ PowerShellEditorServices.sln.ide/storage.ide
 *.jrs
 
 # Don't include PlatyPS generated MAML
-module/PowerShellEditorServices/Commands/en-US/*-help.xml
+module/PowerShellEditorServices/Commands/en-US/*-help.xml
+
+# Visual Studio for Mac generated file
+*.userprefs