Update index.d.ts (#40244)
Update index.d.ts (#40244)

- add ifnotexist to indexConfig
georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,dsebastien/DefinitelyTyped