chore: use nodejs 13 on appveyor to try to fix its build problems (JSZip not found)

.appveyor.yml

@@ -4,7 +4,7 @@ environment:
   matrix:
     # - nodejs_version: "8" 
     # - nodejs_version: "10"
-    - nodejs_version: "12"
+    - nodejs_version: "13"
 platform:
   # - x86
   - x64

src/OpenSheetMusicDisplay/OpenSheetMusicDisplay.ts

@@ -395,7 +395,7 @@ export class OpenSheetMusicDisplay {
         // alternative to if block: this.drawingsParameters.drawCursors = options.drawCursors !== false. No if, but always sets drawingParameters.
         // note that every option can be undefined, which doesn't mean the option should be set to false.
         if (options.drawHiddenNotes) {
-            this.drawingParameters.drawHiddenNotes = true;
+            this.drawingParameters.drawHiddenNotes = true; // not yet supported
         }
         if (options.drawCredits !== undefined) {
             this.drawingParameters.DrawCredits = options.drawCredits; // sets DrawComposer, DrawTitle, DrawSubtitle, DrawLyricist.