[new feature] add sticky component (#3888)

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

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+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`renders demo correctly 1`] = `
+<div>
+  <div>
+    <div>
+      <div class="van-sticky"><button class="van-button van-button--primary van-button--normal" style="margin-left: 15px;"><span class="van-button__text">
+        基础用法
+      </span></button></div>
+    </div>
+  </div>
+  <div>
+    <div style="height: 0px;">
+      <div class="van-sticky van-sticky--fixed" style="top: 50px;"><button class="van-button van-button--info van-button--normal" style="margin-left: 115px;"><span class="van-button__text">
+        吸顶距离
+      </span></button></div>
+    </div>
+  </div>
+  <div>
+    <div style="height: 150px; background-color: rgb(255, 255, 255);">
+      <div>
+        <div class="van-sticky"><button class="van-button van-button--warning van-button--normal" style="margin-left: 215px;"><span class="van-button__text">
+          指定容器
+        </span></button></div>
+      </div>
+    </div>
+  </div>
+</div>
+`;

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

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+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`container prop 1`] = `
+<div style="height: 20px;">
+  <div style="height: 10px;">
+    <div class="van-sticky van-sticky--fixed" style="transform: translate3d(0, -5px, 0);">
+      Content
+    </div>
+  </div>
+</div>
+`;
+
+exports[`container prop 2`] = `
+<div style="height: 20px;">
+  <div style="height: 10px;">
+    <div class="van-sticky" style="">
+      Content
+    </div>
+  </div>
+</div>
+`;
+
+exports[`offset-top prop 1`] = `
+<div style="height: 10px;">
+  <div class="van-sticky van-sticky--fixed" style="top: 10px;">
+    Content
+  </div>
+</div>
+`;
+
+exports[`sticky to top 1`] = `
+<div style="height: 10px;">
+  <div class="van-sticky">
+    Content
+  </div>
+</div>
+`;
+
+exports[`sticky to top 2`] = `
+<div style="height: 10px;">
+  <div class="van-sticky van-sticky--fixed">
+    Content
+  </div>
+</div>
+`;
+
+exports[`z-index prop 1`] = `
+<div style="height: 10px;">
+  <div class="van-sticky van-sticky--fixed" style="z-index: 0;">
+    Content
+  </div>
+</div>
+`;