feat: ability to use another theme

[patzick]

Changes

related: #1158

The feature allows using another theme as a base, by simply putting its name into shopware-pwa.config.js file like this:

module.exports = {
  shopwareEndpoint: 'https://pwa-demo-api.shopware.com',
  shopwareAccessToken: 'SWSC40-LJTNO6COUEN7CJMXKLA',
  // theme: '@shopware-pwa/default-theme',
  theme: 'my-theme',
}

1. It looks by path, so rootDirectory/path
2. If not found it looks in node modules

Preferably theme should be placed in node_modules so published as NPM package.

Next things to do with the task is to add tools for theme building.

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://eb33d71baf2d732b2dc87ed057bcb50144682eec.shopware-pwa-canary.preview.storefrontcloud.io

[mkucmus]

🥇