feat: add firstSlideNum prop of PresentationProps

src/core-interfaces.ts

@@ -1864,6 +1864,7 @@ export interface PresentationProps {
 	subject: string
 	theme: ThemeProps
 	title: string
+	firstSlideNum: number
 }
 // PRIVATE interface
 export interface IPresentationProps extends PresentationProps {

src/gen-xml.ts

@@ -1769,7 +1769,7 @@ export function makeXmlPresentation (pres: IPresentationProps): string {
 	let strXml =
 		`<?xml version="1.0" encoding="UTF-8" standalone="yes"?>${CRLF}` +
 		'<p:presentation xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships" ' +
-		`xmlns:p="http://schemas.openxmlformats.org/presentationml/2006/main" ${pres.rtlMode ? 'rtl="1"' : ''} saveSubsetFonts="1" autoCompressPictures="0">`
+		`xmlns:p="http://schemas.openxmlformats.org/presentationml/2006/main" firstSlideNum="${pres.firstSlideNum}" ${pres.rtlMode ? 'rtl="1"' : ''} saveSubsetFonts="1" autoCompressPictures="0">`
 
 	// STEP 1: Add slide master (SPEC: tag 1 under <presentation>)
 	strXml += '<p:sldMasterIdLst><p:sldMasterId id="2147483648" r:id="rId1"/></p:sldMasterIdLst>'

src/pptxgen.ts

@@ -311,6 +311,15 @@ export default class PptxGenJS implements IPresentationProps {
 		return this._shapes
 	}
 
+	private _firstSlideNum = 1
+	public get firstSlideNum (): number {
+		return this._firstSlideNum
+	}
+
+	public set firstSlideNum (val: number) {
+		this._firstSlideNum = val
+	}
+
 	constructor () {
 		const layout4x3: PresLayout = { name: 'screen4x3', width: 9144000, height: 6858000 }
 		const layout16x9: PresLayout = { name: 'screen16x9', width: 9144000, height: 5143500 }