chore(generate): allow promises in slide.generate callbacks

Author: singerla

__tests__/generate-pptxgenjs-image.test.ts

@@ -1,5 +1,4 @@
 import Automizer from '../src/automizer';
-import { ChartData, modify } from '../src';
 
 test('insert an image with pptxgenjs on a template slide', async () => {
   const automizer = new Automizer({

src/types/types.ts

@@ -238,7 +238,7 @@ export type ImportElement = {
 export type GenerateOnSlideCallback = (
   pptxGenJSSlide: IPptxGenJSSlide,
   pptxGenJS: PptxGenJS,
-) => Promise<void>;
+) => Promise<void> | void;
 export type GenerateElements = {
   objectName?: string;
   tmpSlideNumber?: number;