[new feature] add sticky component (#3888)

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

@@ -0,0 +1,53 @@
+// 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>
+`;