feat(Watermark): support style and customStyle props

Author: anlyyao

packages/components/watermark/watermark.less

@@ -1,5 +1,11 @@
 @import '../common/style/base.less';
 
+.@{prefix}-watermark {
+  position: relative;
+  overflow: hidden;
+  width: 100%;
+}
+
 @keyframes watermark {
   0% {
     left: var(--watermark-left-0);

packages/components/watermark/watermark.wxml

@@ -1,6 +1,6 @@
 <wxs module="_" src="../common/utils.wxs" />
 
-<view style="position: relative; overflow: hidden; width: 100%">
+<view class="{{classPrefix}} class {{prefix}}-class" style="{{_._style([style, customStyle])}}">
   <block wx:if="{{content}}">{{content}}</block>
   <slot name="content" />
   <slot />