added support for 'Preview' element in VSIX Manifest

Author: RalfKoban

Parser/Flavors/XmlFlavorForVsixManifest.cs

@@ -20,6 +20,7 @@ public sealed class XmlFlavorForVsixManifest : XmlFlavor
                                                                             "Dependency",
                                                                             "Asset",
                                                                             "Prerequisite",
+                                                                            "Preview",
                                                                         };
 
         public override bool ParseAttributesEnabled => false;