[breaking change] rebuild style struct (#2021)

packages/tabbar/demo/index.vue

@@ -50,7 +50,7 @@ export default {
 };
 </script>
 
-<style lang="postcss">
+<style lang="less">
 .demo-tabbar {
   .van-tabbar {
     position: relative;

packages/tabbar/index.less

@@ -0,0 +1,14 @@
+@import '../style/var';
+
+.van-tabbar {
+  width: 100%;
+  height: 50px;
+  display: flex;
+  background-color: @white;
+
+  &--fixed {
+    left: 0;
+    bottom: 0;
+    position: fixed;
+  }
+}