Eleventy: Relative includes in Nunjucks

[zachleat]

Unearthed in #189

Nunjucks seems to support this, per https://github.com/mozilla/nunjucks/pull/349

This is already tested as working in Pug and EJS. There are probably other engines that need tests here. Worth testing in Liquid too.

[zachleat]

Also the include_relative Liquidjs feature reported in https://24ways.org/2018/turn-jekyll-up-to-eleventy/ by @paulrobertlloyd

(on second look include_relative doesn’t seem to be available in liquidjs 😅)

[zachleat]

Buncha stuff landed here for this. Tests and docs for Relative Includes are now fully supported in:

• Nunjucks https://www.11ty.io/docs/languages/nunjucks/#supported-features
• Liquid https://www.11ty.io/docs/languages/liquid/#supported-features
• EJS https://www.11ty.io/docs/languages/ejs/#supported-features (was already supported, docs improved)
• Pug https://www.11ty.io/docs/languages/pug/#supported-features (was already supported, docs improved)