[pages-watch-mode] POC

[CarmenPopoviciu]

Enable switching between different Pages projects types (Functions | Advanced Mode | assets-only) in watch mode, without requiring users to restart the dev process.

This is so POC I can't even ;)

Raised in #2553

What this PR solves / how to test

Fixes #[insert GH or internal issue number(s)].

Author has addressed the following

• Tests
  • [ ] TODO (before merge)
  • [ ] Included
  • [ ] Not necessary because:
• E2E Tests CI Job required? (Use "e2e" label or ask maintainer to run separately)
  • [ ] I don't know
  • [ ] Required / Maybe required
  • [ ] Not required because:
• Changeset (Changeset guidelines)
  • [ ] TODO (before merge)
  • [ ] Included
  • [ ] Not necessary because:
• Public documentation
  • [ ] TODO (before merge)
  • [ ] Cloudflare docs PR(s):
  • [ ] Not necessary because: