lib.d.ts: Include undefined in return type of JSON.stringify()

JSON.stringify() may return undefined if the given input value is undefined or if the input value is not serializable (like `function(){}`).
See #18879

src/lib/es5.d.ts

@@ -943,14 +943,14 @@ interface JSON {
       * @param replacer A function that transforms the results.
       * @param space Adds indentation, white space, and line break characters to the return-value JSON text to make it easier to read.
       */
-    stringify(value: any, replacer?: (key: string, value: any) => any, space?: string | number): string;
+    stringify(value: any, replacer?: (key: string, value: any) => any, space?: string | number): string | undefined;
     /**
       * Converts a JavaScript value to a JavaScript Object Notation (JSON) string.
       * @param value A JavaScript value, usually an object or array, to be converted.
       * @param replacer An array of strings and numbers that acts as a approved list for selecting the object properties that will be stringified.
       * @param space Adds indentation, white space, and line break characters to the return-value JSON text to make it easier to read.
       */
-    stringify(value: any, replacer?: (number | string)[] | null, space?: string | number): string;
+    stringify(value: any, replacer?: (number | string)[] | null, space?: string | number): string | undefined;
 }
 
 /**