upgrade to imgui 1.91.0 (#490)

* upgrade to imgui 1.91.0 as well as following changes

- cleaned up Generated folder to ensure there is no legacy stuff
- Moved ImGuiModFlags out of Generated since its not generated
- added a new wellknown type that comes with this release i.e. ImGuiSelectionUserData

* add .net6 back until we migrate to utf8 strings

* release new version

Author: zaafar

deps/cimgui/linux-x64/cimgui.so

@@ -56,6 +56,7 @@ public class TypeInfo
             { "ImPlotGetter", "IntPtr" },
             { "ImPlotTransform", "IntPtr" },
             { "ImGuiKeyChord", "ImGuiKey" },
+            { "ImGuiSelectionUserData", "long" },
         };
 
         public static readonly List<string> WellKnownEnums = new List<string>()