Proof of concept: Shared worker plugins

[novemberborn]

"Shared workers" are code that runs in AVA's main process, that can communicate with test workers.

The idea is that a test file can require a module which registers itself as a plugin. The main process launches a worker thread (so this is a Node.js 12+ feature) which can communicate with all test files.

The worker thread would survive across watcher runs, under the assumption that the worker code came from a third-party and so doesn't need to be reloaded.

Advanced serialization is used for the communication between the test worker, main processes and worker thread. Consequently this requires Node.js 12.16. Though I'm also using some features introduced in Node.js 12.17.

This should give us the low-level primitives for #2048 and #1366.

[novemberborn]

Wow I think I got this working (let's see what CI says). There's a proof of concept here: https://github.com/avajs/ava/tree/shared-worker-plugins/test/shared-workers/proof-of-concept/fixtures

But I'll try and build something real on top of this next.

[novemberborn]

I think I have full code coverage… will try and update my other proof-of-concept packages and then see if I can sneak this into a release.

[novemberborn]

I'm looking to sneak this into a release, but I still have to update the issues, plan work, document things, and ship some example packages…