example: Add an example of drawing a smiley face with canvas

Adapted from https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Drawing_shapes

examples/canvas/index.js

@@ -0,0 +1,5 @@
+// For more comments about what's going on here, check out the `hello_world`
+// example.
+import('./wasm_bindgen_canvas_demo').then(canvas => {
+  canvas.draw();
+});