Combine two Babel and two Webpack configs into one

[simison]

• Webpack
• Babel (done in Build: Share babel config for client and extensions #11811)

Files

• Webpack config for block editor extensions:
  https://github.com/Automattic/jetpack/blob/8bb751954ee883e404c95fe95fb93ce659737a97/webpack.config.extensions.js

• Webpack config for admin dash:
  https://github.com/Automattic/jetpack/blob/8bb751954ee883e404c95fe95fb93ce659737a97/webpack.config.js