BottomSheet: first attempt for animation

src/platform/polyfills.ts

@@ -0,0 +1,8 @@
+//! This module has side effect.
+//! It recommended to include the module by <script> tag.
+if (!document.body.animate) {
+  // @ts-ignore: this file is polyfill, no exposed decls
+  import("web-animations-js").then(() => {
+    console.warn("web animation polyfill is included");
+  });
+}