feat!: (web) (flutter) create canvas when createViewer is called (no longer need to manually add canvas element to web HTML)

thermion_dart/lib/thermion_dart/thermion_viewer.dart

@@ -85,7 +85,6 @@ abstract class ThermionViewer {
   ///
   Future render();
 
-  
   ///
   /// Render a single frame to the viewport and copy the pixel buffer to [out].
   ///