New transforms-esque feature that runs earlier, modifies HTML before inserted in collections/layouts

[zachleat]

https://www.11ty.dev/docs/transforms/

Related: 11ty/eleventy-plugin-rss#47
Related: #3294

This would offer some performance benefit and avoid duplicate transforms execution (via renderTransforms filter and the traditional post-build transforms execution step)

Could be useful for image transforms. Drawback: it would not process the HTML boilerplate in layouts.

[zachleat]

Notably, this is different from the preprocessor API: https://www.11ty.dev/docs/config-preprocessors/