feat(default-theme): new slots on checkout

[mkucmus]

Changes

• normalizes the passed slot name for shipping and billing addresses on the checkout
• adds new slots: shipping step, billing step, and summary for shipping method

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://812e7792361295dde62479e0a1888dc3e3506922.shopware-pwa-canary.preview.storefrontcloud.io