Eleventy Ditches Luxon, Builds Custom RFC 9557 Date Parser

To reduce Eleventy's client-side bundle size and prepare for native Temporal API support, the team decided to replace the Luxon date parsing library with a custom RFC 9557-compliant solution. The new library is smaller, more accurate, and its output matches both the upcoming Temporal API and Luxon, although some breaking changes exist. This ultimately simplifies maintenance and improves performance.