feat(Overlay): add @overlay-z-index less var

src/overlay/index.less

@@ -4,6 +4,7 @@
   position: fixed;
   top: 0;
   left: 0;
+  z-index: @overlay-z-index;
   width: 100%;
   height: 100%;
   background-color: @overlay-background-color;

src/overlay/index.tsx

@@ -57,13 +57,10 @@ function Overlay(
 
 Overlay.props = {
   show: Boolean,
+  zIndex: [Number, String],
   duration: [Number, String],
   className: null as any,
   customStyle: Object,
-  zIndex: {
-    type: [Number, String],
-    default: 1,
-  },
 };
 
 export default createComponent<OverlayProps, OverlayEvents>(Overlay);

src/overlay/test/__snapshots__/demo.spec.js.snap

@@ -3,10 +3,10 @@
 exports[`renders demo correctly 1`] = `
 <div>
   <div><button class="van-button van-button--primary van-button--normal" style="margin-left: 16px;"><span class="van-button__text">显示遮罩层</span></button>
-    <div class="van-overlay" style="z-index: 1; display: none;" name="van-fade"></div>
+    <div class="van-overlay" style="display: none;" name="van-fade"></div>
   </div>
   <div><button class="van-button van-button--primary van-button--normal" style="margin-left: 16px;"><span class="van-button__text">嵌入内容</span></button>
-    <div class="van-overlay" style="z-index: 1; display: none;" name="van-fade">
+    <div class="van-overlay" style="display: none;" name="van-fade">
       <div class="wrapper">
         <div class="block"></div>
       </div>

src/overlay/test/__snapshots__/index.spec.js.snap

@@ -1,11 +1,11 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`class-name prop 1`] = `<div class="van-overlay my-overlay" style="z-index: 1;" name="van-fade"></div>`;
+exports[`class-name prop 1`] = `<div class="van-overlay my-overlay" name="van-fade"></div>`;
 
-exports[`custom style prop 1`] = `<div class="van-overlay" style="z-index: 1; background-color: red;" name="van-fade"></div>`;
+exports[`custom style prop 1`] = `<div class="van-overlay" style="background-color: red;" name="van-fade"></div>`;
 
-exports[`default slot 1`] = `<div class="van-overlay" style="z-index: 1; display: none;" name="van-fade">Custom Default</div>`;
+exports[`default slot 1`] = `<div class="van-overlay" style="display: none;" name="van-fade">Custom Default</div>`;
 
-exports[`duration prop 1`] = `<div class="van-overlay" style="z-index: 1; animation-duration: 1s;" name="van-fade"></div>`;
+exports[`duration prop 1`] = `<div class="van-overlay" style="animation-duration: 1s;" name="van-fade"></div>`;
 
 exports[`z-index prop 1`] = `<div class="van-overlay" style="z-index: 99;" name="van-fade"></div>`;

src/password-input/test/__snapshots__/demo.spec.js.snap

@@ -16,7 +16,7 @@ exports[`renders demo correctly 1`] = `
       </ul>
       <div class="van-password-input__info">密码为 6 位数字</div>
     </div>
-    <div class="van-number-keyboard van-number-keyboard--default van-number-keyboard--safe-area-inset-bottom" style="z-index: 100;" name="van-slide-up">
+    <div class="van-number-keyboard van-number-keyboard--default van-number-keyboard--safe-area-inset-bottom" name="van-slide-up">
       <div class="van-number-keyboard__body"><i role="button" tabindex="0" class="van-hairline van-key">1</i><i role="button" tabindex="0" class="van-hairline van-key">2</i><i role="button" tabindex="0" class="van-hairline van-key">3</i><i role="button" tabindex="0" class="van-hairline van-key">4</i><i role="button" tabindex="0" class="van-hairline van-key">5</i><i role="button" tabindex="0" class="van-hairline van-key">6</i><i role="button" tabindex="0" class="van-hairline van-key">7</i><i role="button" tabindex="0" class="van-hairline van-key">8</i><i role="button" tabindex="0" class="van-hairline van-key">9</i><i role="button" tabindex="0" class="van-hairline van-key van-key--gray van-key--extra"></i><i role="button" tabindex="0" class="van-hairline van-key">0</i><i role="button" tabindex="0" class="van-hairline van-key van-key--gray van-key--delete">删除</i></div>
     </div>
   </div>

src/style/var.less

@@ -471,6 +471,7 @@
 @number-keyboard-z-index: 100;
 
 // Overlay
+@overlay-z-index: 1;
 @overlay-background-color: rgba(0, 0, 0, 0.7);
 
 // Pagination