Started to rewrite CKEDITOR placeholder plugin for CKEDITOR5.

assets/ckeditor/plugins/PartDBLabel/PartDBLabelCommand.js

@@ -0,0 +1,31 @@
+import Command from '@ckeditor/ckeditor5-core/src/command';
+
+export default class PartDBLabelCommand extends Command {
+    execute( { value } ) {
+        const editor = this.editor;
+        const selection = editor.model.document.selection;
+
+        editor.model.change( writer => {
+            // Create a <placeholder> elment with the "name" attribute (and all the selection attributes)...
+            const placeholder = writer.createElement( 'partdb_label', {
+                ...Object.fromEntries( selection.getAttributes() ),
+                name: value
+            } );
+
+            // ... and insert it into the document.
+            editor.model.insertContent( placeholder );
+
+            // Put the selection on the inserted element.
+            writer.setSelection( placeholder, 'on' );
+        } );
+    }
+
+    refresh() {
+        const model = this.editor.model;
+        const selection = model.document.selection;
+
+        const isAllowed = model.schema.checkChild( selection.focus.parent, 'partdb_label' );
+
+        this.isEnabled = isAllowed;
+    }
+}