[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

.github/actions/spell-check/excludes.txt

@@ -26,6 +26,7 @@ ignore$
 \.pdf$
 \.PNG$
 \.png$
+\.stl$
 \.woff$
 \.zip$
 ^doc/devdocs/akaLinks\.md$