style: decentralized less var (#8369)

* style: decentralized less var

* chore: remove emtpy var file

src/dropdown-item/index.less

@@ -1,4 +1,5 @@
-@import '../style/var.less';
+@import './var.less';
+@import '../dropdown-menu/var.less';
 
 .van-dropdown-item {
   position: fixed;

src/dropdown-item/var.less

@@ -0,0 +1,3 @@
+@import '../style/var.less';
+
+@dropdown-item-z-index: 10;