add createLevel method to AbstractNumbering

ts/numbering/abstract-numbering.ts

@@ -35,6 +35,12 @@ export class AbstractNumbering extends XmlComponent {
         this.root.push(level);
     }
 
+    public createLevel(num: number, format: string, text: string, align: string) {
+        const level = new Level(num, format, text, align);
+        this.addLevel(level);
+        return level;
+    }
+
     public clearVariables(): void {
         _.forEach(this.root, (element) => {
             element.clearVariables();