feat: add frontend link rewriter to propagate brand parameter

[rbcorrales]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

• Add Link_Rewriter class to enqueue a frontend script that rewrites internal links.
• Add new link-rewriter/index.js script:
  • Detects current brand from URL (?brand=brand-slug).
  • Rewrites all internal anchor tags to include the brand parameter if not already present.
  • Skips external links and links that already have a brand parameter.
• Register and enqueue the script via Link_Rewriter::init() in Initializer.
• Update webpack config to bundle linkRewriter and provide WordPress externals.

This helps preserve brand context through links as users navigate the site.

How to test the changes in this Pull Request:

1. Verify that the Link Rewriter is initialized and its script is enqueued on the frontend.
2. Verify that internal links on a page with a brand override are rewritten.
   • Visit a public page (for example, a post or an archive) and append ?brand=brand‑a (or a valid brand slug) to the URL.
   • Inspect the page (using your browser's dev tools) and locate internal anchor tags (for example, a link to another post or a category).
   • Confirm that the href attribute of these internal links is rewritten (i.e. ?brand=brand‑a is appended).
3. Verify that links that are not internal are not rewritten.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[adekbadek]

Is this necessary, if these scripts are listed as dependencies in the call on line 37?