Allow configuration to return a promise (async-friendly)

[zachleat]

More information at #612 by @Munter

I attempted to use an async function in .eleventy.js because I wanted to try something depending on disk access, and usually do those things async. It turned out everything I tried to configure didn't work. The reason for this was that the config resolution doesn't handle async or promises, and just went ahead and merged my returned promise with the default config.

Right now this isn’t possible, this issue suggests that it should be:

module.exports = async function(eleventyConfig) {};

The problem here is that the config is used in constructors all over the code base (which are not async friendly).

[zachleat]

This repository is now using lodash style issue management for enhancements. This means enhancement issues will now be closed instead of leaving them open.

View the enhancement backlog here. Don’t forget to upvote the top comment with 👍!

[zachleat]

If I could vote twice I’d vote again since this bit me just now (the error message that @Munter added was so good though)

[zachleat]

This was attempted in #1429 but didn’t make it in (it was my fault)

[zachleat]

This is tied to ESM support, which includes top-level await (milestoning to 3.0 and removing from the enhancement queue)

[zachleat]

Fixed in PR #3074