add DateTime class

[drdzyk]

This PR adds some functionality of php DateTime class. Namelly, the following functions:

class DateTime {
  /** @kphp-extern-func-info can_throw */
  public function __construct(string $datetime = "now", ?DateTimeZone $timezone = null);
  public static function createFromFormat(string $format, string $datetime, ?DateTimeZone $timezone = null): ?DateTime;
  public static function getLastErrors(): array|false;
  public function modify(string $modifier): ?DateTime;
  public function setDate(int $year, int $month, int $day): DateTime;
  public function setISODate(int $year, int $week, int $dayOfWeek = 1): DateTime;
  public function setTime(
      int $hour,
      int $minute,
      int $second = 0,
      int $microsecond = 0
  ): DateTime;
  public function setTimestamp(int $timestamp): DateTime;
  public function format(string $format): string;
  public function getOffset(): int;
  public function getTimestamp(): int;
}