Fix: Add Promise as return type to save method (#3272)

Co-authored-by: Uzlopak <aras.abbasi@googlemail.com>

Author: malthoff

types/index.d.ts

@@ -844,7 +844,8 @@ declare module "jspdf" {
  ry: number,
  style?: string | null
  ): jsPDF;
- save(filename?: string, options?: { returnPromise?: boolean }): jsPDF;
+ save(filename: string, options: { returnPromise: true }): Promise<void>;
+ save(filename?: string): jsPDF;
  saveGraphicsState(): jsPDF;
  setCharSpace(charSpace: number): jsPDF;
  setCreationDate(date?: Date | string): jsPDF;