Fix postcss 8.x compatibility

[c0rdyc3p5]

Update postcss to be compatible with version 8, options are now useable too.

Fixes #344

[SimonDesautels]

This might fix #344

[kevinaxial]

@MartijnCuppens bump

[brennanangel]

Perhaps @XhmikosR could review?

[XhmikosR]

Please revert any unrelated changes and keep only the minimum ones needed to fix the problem.

[c0rdyc3p5]

@XhmikosR not too sure if you got the notification, but I made the changes ! :)

[XhmikosR]

Hey!

I was notified, but my concern is that it seems our tests don't cover everything. :/ I mean, shouldn't the existent tests be broken on main already?

Also, looking at https://evilmartians.com/chronicles/postcss-8-plugin-migration it seems to list more changes.

[c0rdyc3p5]

@XhmikosR Yo ! Just checked the others changes needed in the post. By the way, yes the library still works perfectly fine for now, it seems like postcss don't break yet, but with my changes postcss won't output a message error neither (this is the reason why when you do the test, they still output the same). My fix was really just to use the 'new' way and remove the deprecation notice from postcss because in fact, it is 'not' broken, it is just a 'deprecation notice'. When I get free time I'll check to add the others changes if they apply :)

[c0rdyc3p5]

@XhmikosR hi ! Do you have an idea when the changes will roll into a new release ? So we can update our npm package ?

[XhmikosR]

Not sure TBH, because it's been too long and I don't even recall the changed compared to 9.x.

/CC @mdo we might need to prioritize this and I can cut a v10 with what we have now. I just need help with the release notes apart from:

• Node.js >= 12 is required
• postcss 8.x support (unsure if postcss 7 works anymore)
• we now throw an error with invalid options

[cfxd]

@XhmikosR any update on when this will be released? Just would love to get it working with PostCss 8.x! Please lmk if there's anything I can do to get a release out there!

[XhmikosR]

Sorry for the so late reply. So, I'd like to tackle this for main at least.

That being said, I don't even recall the current main changes plus it seems we don't list postcss in peerDependencies but that's how it is since even AFAICT.

I'll try to cut a new major release in the next days from main, but the other older versions will be left as is.

[XhmikosR]

I pushed a few updates and now only the release notes are left to handle. Hopefully I'll get some help and cut v10 major release today/tomorrow.

[XhmikosR]

Just released v10 https://github.com/twbs/rfs/releases/tag/v10.0.0

The release notes are far from perfect, so if anyone has any suggestions let me know.