[FEATURE] keep classes of h3,h4

src/keepcustomclasses.js

-import Plugin from "@ckeditor/ckeditor5-core/src/plugin";
+import Plugin from '@ckeditor/ckeditor5-core/src/plugin';
 
 export class KeepCustomClasses extends Plugin {
 	init() {
@@ -6,24 +6,32 @@ export class KeepCustomClasses extends Plugin {
 
 		// Define on which elements the CSS classes should be preserved:
 		setupCustomClassConversion( 'h2', 'heading1', editor );
+		setupCustomClassConversion( 'h3', 'heading2', editor );
+		setupCustomClassConversion( 'h4', 'heading3', editor );
 		setupCustomClassConversion( 'p', 'paragraph', editor );
 
 		// Define custom attributes that should be preserved.
 		setupCustomAttributeConversion( 'h2', 'heading1', 'id', editor );
+		setupCustomAttributeConversion( 'h3', 'heading2', 'id', editor );
+		setupCustomAttributeConversion( 'h4', 'heading3', 'id', editor );
 		setupCustomAttributeConversion( 'p', 'paragraph', 'id', editor );
 
-
 		function setupCustomClassConversion( viewElementName, modelElementName, editor ) {
 			// The 'customClass' attribute will store custom classes from the data in the model so schema definitions allow this attribute.
 			editor.model.schema.extend( modelElementName, { allowAttributes: [ 'customClass' ] } );
 
-            // Define upcast converters for the <img> and <table> elements with a "low" priority so they are run after the default converters.
-            editor.conversion.for( 'upcast' ).add( upcastCustomClasses( viewElementName ), { priority: 'low' } );
-
-            // Define downcast converters for a model element with a "low" priority so they are run after the default converters.
-            // Use `downcastCustomClassesToFigure` if you'd like to keep your classes on <figure> element or `downcastCustomClassesToChild` if you'd like to keep your classes on a <figure> child element, i.e. <img>.
-            editor.conversion.for( 'downcast' ).add( downcastCustomClassesToFigure( modelElementName ), { priority: 'low' } );
-            // editor.conversion.for( 'downcast' ).add( downcastCustomClassesToChild( viewElementName, modelElementName ), { priority: 'low' } );
+			// Define upcast converters for the <img> and <table> elements with a
+			// "low" priority so they are run after the default converters.
+			editor.conversion.for( 'upcast' ).add( upcastCustomClasses( viewElementName ),
+				{ priority: 'low' } );
+
+			// Define downcast converters for a model element with a
+			// "low" priority so they are run after the default converters.
+			// Use `downcastCustomClassesToFigure` if you'd like to keep your classes on
+			// <figure> element or `downcastCustomClassesToChild` if you'd like to keep your
+			// classes on a <figure> child element, i.e. <img>.
+			editor.conversion.for( 'downcast' ).add( downcastCustomClassesToFigure( modelElementName ),
+				{ priority: 'low' } );
 		}
 
 		function setupCustomAttributeConversion( viewElementName, modelElementName, viewAttribute, editor ) {
@@ -33,7 +41,8 @@ export class KeepCustomClasses extends Plugin {
 			editor.model.schema.extend( modelElementName, { allowAttributes: [ modelAttribute ] } );
 
 			editor.conversion.for( 'upcast' ).add( upcastAttribute( viewElementName, viewAttribute, modelAttribute ) );
-            editor.conversion.for( 'downcast' ).add( downcastAttribute( modelElementName, viewElementName, viewAttribute, modelAttribute ) );
+			editor.conversion.for( 'downcast' ).add( downcastAttribute( modelElementName, viewElementName,
+				viewAttribute, modelAttribute ) );
 		}
 
 		function upcastCustomClasses( elementName ) {
@@ -121,4 +130,4 @@ export class KeepCustomClasses extends Plugin {
 			} );
 		}
 	}
-  }
+}
\ No newline at end of file