[FileExplorer] StlThumbnailProvider (#15568)

* Adds StlThumbnailProvider

* Spell checker fixes

* Adds missing changes

* Attempts to fix alpha background issue

* Adds missing dependency references

* Upgrades .NET Core 3.1 to .NET 5

* Updates Helix Toolkit to fix .net5 compatibility

* Return null bitmap If STL model is empty

src/settings-ui/Settings.UI/Views/PowerPreviewPage.xaml

@@ -77,6 +77,13 @@
                                           x:Uid="ToggleSwitch"/>
                         </controls:Setting.ActionContent>
                     </controls:Setting>
+
+                  <controls:Setting x:Uid="FileExplorerPreview_ToggleSwitch_STL_Thumbnail" Icon="&#xF158;">
+                      <controls:Setting.ActionContent>
+                          <ToggleSwitch IsOn="{x:Bind Mode=TwoWay, Path=ViewModel.STLThumbnailIsEnabled}"
+                                        x:Uid="ToggleSwitch"/>
+                      </controls:Setting.ActionContent>
+                  </controls:Setting>
                 </controls:SettingsGroup>
                 
             </StackPanel>