[new feature] add CountDown component (#3805)

types/index.d.ts

@@ -17,6 +17,7 @@ export class Button extends VanComponent {}
 export class Card extends VanComponent {}
 export class Cell extends VanComponent {}
 export class CellGroup extends VanComponent {}
+export class CountDown extends VanComponent {}
 export class Divider extends VanComponent {}
 export class SwipeCell extends VanComponent {}
 export class Checkbox extends VanComponent {}