Extract some core features to official plugins

[uncenter]

In addition to #3647, I think Eleventy should avoid including opinionated and complicated features like content dates and tags to core. Tags create issues like #1791 (see also #1254, which proposes removing this feature as well), content dates lead to us relying on the large Luxon library (#3587), etc. This kind of functionality would be better suited to plugins; @11ty/eleventy-plugin-tags and @11ty/eleventy-plugin-content-dates.