Move to `postcss-load-config` 5.0

[ai]

This project is using postcss-load-config 3.0, but the latest version is 5.0.

In new version we removed Node.js 12—16, improved ESM and TS support, and moved to async API.

[ota-meshi]

ESLint currently needs to work synchronously, so we can't use the new version of postcss-load-config where the sync API has been dropped 😓

[ai]

It was dropped because of ESM/TS support where we have only async API.

Is it possible to preload config by async API in some initializer before ESLint rules call?

[ota-meshi]

I've been looking for it many times but can't find it.
Probably not supported.
eslint/eslint#15394

If we need to upgrade postcss-load-config right now, I think we'll probably need to use something like synckit to synchronize it.
https://github.com/un-ts/synckit

[JounQin]

Is there anything blocked? I believe @ota-meshi is very similar with synckit, and if you agree I can raise a PR for it.

[ota-meshi]

I'm sure that using synckit could solve the problem, but I don't have the time to implement it right now 😓

[JounQin]

I'm sure that using synckit could solve the problem, but I don't have the time to implement it right now 😓

and if you agree I can raise a PR for it.

I'll raise a PR for it.

[JounQin]

@ota-meshi Here you are. #665