Add flag to run event handlers sequentially

[vrugtehagel]

Resolves #3415.

This PR adds a flag to run event handlers sequentially, instead of in parallel. This is to avoid issues with plugins interfering with one another if they are doing asynchronous operations on the same set of files in parallel.

[zachleat]

Looking great! I definitely don’t want to add a temporary CLI argument for this though—I think it’d be better to keep it as a Configuration API only.

[zachleat]

I think just revert the changes in cmd.cjs and src/Eleventy.js and we’ll be good

[vrugtehagel]

Alright, @zachleat - removed the CLI flag entirely and moved the setEmitterMode method to UserConfig. So essentially, now users would have the ability to do

export default function(eleventyConfig){
    eleventyConfig.setEmitterMode('sequential');
    // or…
    eleventyConfig.events.setHandlerMode('sequential');
} 

Hypothetically, we can remove the former in favor of the latter, but perhaps it's nice to have this "top-level" method since we have it for almost everything else. Let me know what you think :)

[zachleat]

@vrugtehagel yeah I think in docs we’ll lean towards only recommending the former method—thank you!

[zachleat]

LGTM thank you!

[zachleat]

Shipping with 3.0.0-beta.2 and 3.0.0-alpha.19

[zachleat]

Temporary docs preview deploying here: https://11ty-website-git-v3-11ty.vercel.app/docs/events/#emitter-modes