Fix API under exactOptionalPropertyTypes (#48505)

src/compiler/types.ts

@@ -6613,7 +6613,7 @@ namespace ts {
         realpath?(path: string): string;
         getCurrentDirectory?(): string;
         getDirectories?(path: string): string[];
-        useCaseSensitiveFileNames?: boolean | (() => boolean);
+        useCaseSensitiveFileNames?: boolean | (() => boolean) | undefined;
     }
 
     /**