Add source code addon to Storybook

[MAILPOET-2769]

.storybook/main.js

@@ -1,3 +1,5 @@
+const path = require('path');
+
 module.exports = {
   stories: ['../assets/js/src/**/_stories/*.tsx'],
   addons: [
@@ -5,5 +7,17 @@ module.exports = {
     '@storybook/addon-links',
     '@storybook/preset-typescript',
     'storybook-addon-performance/register',
+    {
+      name: '@storybook/addon-storysource',
+      options: {
+        rule: {
+          test: [/_stories\/.*\.tsx?$/],
+          include: [path.resolve(__dirname, '../assets/js/src')],
+        },
+        loaderOptions: {
+          parser: 'typescript',
+        },
+      },
+    },
   ],
 };