fix(ui/date.buildDate): date.buildDate returns Date and not string (quasarframework#7888)

* date.buildDate returns Date and not string

According to https://github.com/quasarframework/quasar/blob/af3ef42fa2dda2debaeb1a45675cead87b26f09b/ui/src/utils/date.js#L355

Author: anthony-o

ui/types/utils/date.d.ts

@@ -26,7 +26,7 @@ export type DateUnitOptions = "second" | "minute" | "hour" | "day" | "month" | "
 export namespace date {
   function isValid(date: number | string): boolean;
   function extractDate(str: string, mask: string, locale?: DateLocale): Date;
-  function buildDate(options: BuildDateOptions, utc?: boolean): string;
+  function buildDate(options: BuildDateOptions, utc?: boolean): Date;
   function getDayOfWeek(date: Date): number;
   function getWeekOfYear(date: Date | number | string): number;
   function isBetweenDates(date: Date | number | string, from: Date | number | string, to: Date | number | string, opts?: { inclusiveFrom: boolean; inclusiveTo: boolean; onlyDate: boolean }): boolean;