feat$(abcjs): add extension file

src/extensions/abcNotationExtension.js

@@ -0,0 +1,13 @@
+// import abcjs from 'abcjs';
+import markdownItNotesSheet from './libs/markdownItNotesSheet';
+import extensionSvc from '../services/extensionSvc';
+
+extensionSvc.onGetOptions((options, properties) => {
+  options.abc = properties.extensions.abc.enabled;
+});
+
+extensionSvc.onInitConverter(2, (markdown, options) => {
+  if (options.abc) {
+    markdown.use(markdownItNotesSheet);
+  }
+});