Drop Node 10 and 12. Upgrade to eslint 8

[benmccann]

ref sveltejs/kit#4553

[Conduitry]

@benmccann @dummdidumm Do you think we also want to bump the peerdep of ESLint to 8+? I would think so, but I wanted to get other opinions, since it wasn't included in this PR.

[benmccann]

I don't have much of an opinion one way or the other. The one thing to possibly check would be whether eslint 8 is compatible with older versions of Node and would run on sveltejs/svelte repo if there's a concern about being able to run the latest version of this plugin on that repo

[benmccann]

I just bumped the peerdep version. I sent sveltejs/svelte#7506 which shows eslint 8 works just fine on older versions of node.

I also noticed the engines field present in this package and removed it. Saying we support Node 10 was no longer accurate, but making it Node 14 seemed like it could be unnecessarily annoying since this package does actually work on Node 10 and 12 as far as I'm aware

[Conduitry]

If we're saying the peerdep is ESLint 8+ then I suppose we also automatically inherit its engines requirement. As long as this plugin works wherever the underlying ESLint versions we support work, I think we're good there.

[Conduitry]

I've updated the readme to refer to ESLint 8+. I've also updated the devdep range for ESLint to from ^8 to >=8. This makes CI more brittle, but I think it's more important to learn about breaking changes in ESLint 9+.

[Conduitry]

And I just added Node 18 to the CI matrix. I think this is ready to go. I'll merge it, and then release 4.0.0.