sets focus to a thumb of Slider
focus(extra?: boolean): void;
extra: boolean - optional, if the range mode is activated, true will set focus to the second thumb
// sets focus to the thumb;
// if the range mode is activated, sets focus to the first thumb
// sets focus to the second thumb if the range mode is activated
The extra parameter can be used when a range mode of Slider is activated.
added in v7.2