feat(scroll): user-defined scrollbar length using LV_STYLE_LENGTH (#7306)

Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>

examples/scroll/lv_example_scroll_4.c

@@ -39,6 +39,7 @@ void lv_example_scroll_4(void)
     static lv_style_t style;
     lv_style_init(&style);
     lv_style_set_width(&style, 4);      /*Width of the scrollbar*/
+    lv_style_set_length(&style, 20);    /*Length of the scrollbar*/
     lv_style_set_pad_right(&style, 5);  /*Space from the parallel side*/
     lv_style_set_pad_top(&style, 5);    /*Space from the perpendicular side*/