feat: clear btn in web

web/index.html

@@ -45,6 +45,7 @@
         <button id="token2">Token btn 2</button>
         <button id="colorChangeBtnRed">Color btn Red</button>
         <button id="colorChangeBtnBlue">Color btn Blue</button>
+        <button id="clearBtn">Clear</button>
         <section class="contents">
             <article>
                 <div id="flutter_target"></div>