feat(cli): support for creating custom themes

[patzick]

Changes

related #1158

Adds CLI methods to help developing theme:

• shopware-pwa create-theme - interactively prepare a new theme for your repo
• shopware-pwa dev-theme - used internally in theme for development
• shopware-pwa build-theme - used internally to build the theme

Next things to do:

1. prepare base theme
2. refactor default-theme to use base-theme
3. prepare docs on how to create a new theme

Checklist

• the list of features is updated (if it's a feature and it's relevant)
• I followed contributing guidelines

[vercel]

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[github-actions]

💙 shopware-pwa-canary successfully deployed at https://0aee90d7f14eac4f5d06fe2f2f881b4118bea350.shopware-pwa-canary.preview.storefrontcloud.io

[Malwurf]

Awesome 💻