fix(tag): 补充缺少的样式变量，修复在线调试问题 (#635)

src/packages/tag/tag.scss

 @import '../icon/icon.scss';
 .nut-tag {
-  display: inline;
   padding: 0 4px;
-  border-radius: 4px;
+  display: inline-flex;
+  justify-content: center;
+  align-items: center;
+  font-size: $tag-font-size;
+  border-radius: $tag-default-border-radius;
+  height: $tag-height;
   ._icon {
     vertical-align: middle;
     margin-left: 4px;