Add checkout to saleor-platform

[peelar]

The goal of saleor-platform, as far as I know, is to enable booting up all the Saleor applications with one command.

Since we are introducing new applications to the Saleor suite, we should think about adding them to the saleor-platform as well.

Right now, we only boot up react-storefront on port 3001, but the application also contains:

• apps/saleor-app-checkout: http://localhost:3001/
• packages/checkout-storefront: http://localhost:3002/
• packages/ui-kit storybook: http://localhost:6006/