style: decentralized less var (#8369)

* style: decentralized less var

* chore: remove emtpy var file

src/count-down/index.less

@@ -1,4 +1,4 @@
-@import '../style/var.less';
+@import './var.less';
 
 .van-count-down {
   color: @count-down-text-color;

src/count-down/var.less

@@ -0,0 +1,5 @@
+@import '../style/var.less';
+
+@count-down-text-color: @text-color;
+@count-down-font-size: @font-size-md;
+@count-down-line-height: @line-height-md;