Upgrade to storybook 6: the config part

Related issues:
- Removed @babel/plugin-proposal-class-properties plugin. Not sure this is the right thing to do, I'll try to find another workaround
storybookjs/storybook#12093
- Added some core-js magic things in the storybook webpack configuration.
storybookjs/storybook#11255 (comment)

.babelrc

@@ -13,7 +13,6 @@
   ],
   "plugins": [
     "@babel/plugin-transform-react-jsx",
-    "@babel/plugin-proposal-class-properties",
     [
       "module-resolver",
       {

.storybook/main.js

@@ -20,10 +20,14 @@ module.exports = {
         },
       ],
     })
+
     config.resolve.extensions.push('.ts', '.tsx')
+    // https://github.com/storybookjs/storybook/issues/11255#issuecomment-713083086
+    config.resolve.alias['core-js/modules'] = '@storybook/core/node_modules/core-js/modules'
+    config.resolve.alias['core-js/features'] = '@storybook/core/node_modules/core-js/features'
     return config
   },
-  stories: ['../src/**/story.(tsx|mdx)', '../src/**/*.story.(tsx|mdx)'],
+  stories: ['../src/**/*.story.mdx'],
   addons: [
     '@storybook/addon-knobs',
     '@storybook/addon-actions',