style: decentralized less var (#8369)

* style: decentralized less var

* chore: remove emtpy var file

src/tree-select/index.less

@@ -1,4 +1,4 @@
-@import '../style/var.less';
+@import './var.less';
 
 .van-tree-select {
   position: relative;

src/tree-select/var.less

@@ -0,0 +1,10 @@
+@import '../style/var.less';
+
+@tree-select-font-size: @font-size-md;
+@tree-select-nav-background-color: @background-color;
+@tree-select-content-background-color: @white;
+@tree-select-nav-item-padding: 14px @padding-sm;
+@tree-select-item-height: 48px;
+@tree-select-item-active-color: @red;
+@tree-select-item-disabled-color: @gray-5;
+@tree-select-item-selected-size: 16px;