style: decentralized less var (#8369)

* style: decentralized less var

* chore: remove emtpy var file

src/pull-refresh/index.less

@@ -1,4 +1,4 @@
-@import '../style/var.less';
+@import './var.less';
 
 .van-pull-refresh {
   overflow: hidden;

src/pull-refresh/var.less

@@ -0,0 +1,6 @@
+@import '../style/var.less';
+
+@pull-refresh-head-height: 50px;
+@pull-refresh-head-font-size: @font-size-md;
+@pull-refresh-head-text-color: @gray-6;
+@pull-refresh-loading-icon-size: 16px;