feat(area): add area demo (#1415)

* feat(area): add area demo

* chore: simplify demo code

Author: hustcc

examples/line/multiple/demo/marker-active.ts

@@ -17,7 +17,7 @@ fetch('https://gw.alipayobjects.com/os/bmw-prod/55424a73-7cb8-4f79-b60d-3ab627ac
       '#FF99C3',
     ];
 
-    const container = document.getElementById('container');
+    const container = 'container';
     const containerBox = container.getBoundingClientRect();
     const series = uniq(data.map((d) => d.category));
     const markerSize = 6;

examples/pie/basic/demo/custom-label.ts

@@ -5,7 +5,7 @@ const data = [
   { sex: '女', sold: 0.55 },
 ];
 
-const piePlot = new Pie(document.getElementById('container'), {
+const piePlot = new Pie('container', {
   width: 400,
   height: 300,
   appendPadding: 10,