Add subdomain redirect support

[shigahi]

Summary

Add support for configuring subdomain redirects, allowing users to redirect subdomains (e.g., www) to the main domain or vice versa.

Use Cases

1. Redirect www.example.com to example.com
2. Redirect example.com to www.example.com
3. Redirect old subdomains to new locations

Changes

• Add SubdomainRedirect interface with subdomain and redirectUrl fields
• Add "Subdomain Redirects" section in Advanced Settings with add/remove functionality
• Generate SUBDOMAIN_REDIRECTS configuration in Worker script
• Implement redirect logic in fetchAndApply function that:
  • Checks if the request is for a subdomain of the main domain
  • Returns a 301 redirect if a matching redirect is configured
  • Preserves the pathname and query string in the redirect

Configuration Example

const SUBDOMAIN_REDIRECTS = {
  'www': 'https://example.com',
  'old': 'https://example.com/new-location',
};

Benefits

• SEO consolidation (avoid duplicate content on www vs non-www)
• Flexible domain management
• Clean redirect handling with 301 status code

Closes #15