feat(theme-base): theme base package as a foundation for custom themes

[patzick]

Changes

related #1158

prepared theme-base package as a base for all themes.

Next things to do:

1. refactor default-theme -> change structure to match theme building and publishing structure
2. create docs with instruction on how to develop custom 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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🔍 Inspect: https://vercel.com/shopware-pwa/shopware-pwa-docs/lpl0e51ry
✅ Preview: https://shopware-pwa-docs-git-fear-theme-base-functionality.shopware-pwa.vercel.app

[github-actions]

💙 shopware-pwa-canary successfully deployed at https://4f15560ea8733873473abaf2152500d121c6fef4.shopware-pwa-canary.preview.storefrontcloud.io

[mkucmus]

works really great! 🧙‍♂️