refactor: use closure to create multistore extension

[WojtekTheWebDev]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSDoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

[CHANGED] We standardized the way of creating and configuring multistore extension.
Previously, the extension was created by importing multistoreExtension from @vue-storefront/multistore and passing it to the extensions function.
Configuration was passed to the extension by adding multistore property to the configuration object.
Now, the extension is created by calling createMultistoreExtension from @vue-storefront/multistore and passing the multistore configuration to it.

- import { multistoreExtension } from "@vue-storefront/multistore";
+ import { createMultistoreExtension } from "@vue-storefront/multistore";
import { multistoreConfig } from "./multistore.config";

export default {
  integrations: {
    sapcc: {
      location: "@vue-storefront/sapcc-api/server",
      configuration: {
        // ...
-        multistore: multistoreConfig,
      },
      extensions: (predefinedExtensions) => [
        ...predefinedExtensions,
-        multistoreExtension,
+        createMultistoreExtension(multistoreConfig),
      ],
    },
  },
};

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[changeset-bot]

🦋 Changeset detected

Latest commit: af36087

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@vue-storefront/multistore	Major

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[sethidden]

the nits are on code that you didn't write so feel free to just merge without addressing them