feat: all routes collection

[jurgenbelien]

TODO: this implementation for creating a union of multiple collections breaks the preview subscription at the moment

Changes

• Adds helpful generics for authoring content collections.
• Adds a function to automatically create a union of arbitrary route-based collections

Associated issue

Resolves #166

How to test

1. Open preview link
2. Navigate to home and other pages
3. Verify that pages laod

Checklist

• I have performed a self-review of my own code
• I have made sure that my PR is easy to review (not too big, includes comments)
• I have made updated relevant documentation files (in project README, docs/, etc)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• I have notified a reviewer

[cloudflare-workers-and-pages]

Deploying head-start with    Cloudflare Pages

Latest commit:	98b6ce6
Status:	✅  Deploy successful!
Preview URL:	https://ae03152d.head-start.pages.dev
Branch Preview URL:	https://feat-all-routes-collection.head-start.pages.dev

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