feat(examples): activate the storybook source addon (#75)

This replicates what mxgraph examples previously provided: the source code directly accessible within the example.
It allows copy/paste for reuse in user applications and should improve the adoption.
The code used to manage the controls is easily identifiable and can help highlight certain parts of the example (ContextMenu, RubberBand, ...).

Also bump storybook from 6.4.0 to 6.4.13.

packages/html/.storybook/main.js

@@ -5,7 +5,8 @@ module.exports = {
   ],
   "addons": [
     "@storybook/addon-links",
-    "@storybook/addon-essentials"
+    "@storybook/addon-essentials",
+    "@storybook/addon-storysource"
   ],
   staticDirs: ['../public'],
 }

packages/html/package.json

@@ -13,10 +13,11 @@
   },
   "devDependencies": {
     "@babel/core": "^7.16.0",
-    "@storybook/addon-actions": "^6.4.0",
-    "@storybook/addon-essentials": "^6.4.0",
-    "@storybook/addon-links": "^6.4.0",
-    "@storybook/html": "^6.4.0",
+    "@storybook/addon-actions": "^6.4.13",
+    "@storybook/addon-essentials": "^6.4.13",
+    "@storybook/addon-links": "^6.4.13",
+    "@storybook/addon-storysource": "^6.4.13",
+    "@storybook/html": "^6.4.13",
     "babel-loader": "^8.2.3",
     "html-webpack-plugin": "^5.5.0",
     "typescript": "^4.5.2",