fix(controls): improve focus style to meet WCAG 2.2 AA

[dancormier]

Closes STACKS-542

This PR updates focus styles for buttons that are editor controls.

Note
This PR removes styling for .s-btn-group__radio. This element no longer exists (it's now .s-btn-group--radio).

Note
This output of this PR may look incorrect until the @StackExchange/stacks dependency is updated beyond 2.0.9.

[netlify]

✅ Deploy Preview for stacks-editor ready!

Name	Link
🔨 Latest commit	b5634a9
🔍 Latest deploy log	https://app.netlify.com/sites/stacks-editor/deploys/65c3bc23148cfd00082d82f3
😎 Deploy Preview	https://deploy-preview-267--stacks-editor.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[giamir]

@dancormier What do you think of adding @stackoverflow/stacks in the peerDependencies field of the package.json of this library?

"peerDependencies": {
    "@stackoverflow/stacks": "~2.1.0",
    "highlight.js": "^11.6.0"
}

This reference will then be moved to the devDependencies object the same way we do for highlight.js.

I am suggesting this because the release containing the changes on this PR (probably v0.10.0) will rely on the styles of Stacks Classic v2.1.0 or above being rendered alongside the editor. Consumers failing to do so will potentially get a half broken editor. I also think that dependabot (for example in Core) will be smart enough and attempt to bump both libraries at once (worst case it will correctly fail CI when trying to run npm i updating exclusively the editor).

[giamir]

@dancormier What do you think of adding @stackoverflow/stacks in the peerDependencies field of the package.json of this library?

I took the liberty to cut a stacks classic release candidate and also add 2.1.0 as peer deps in this PR.
I hope that's ok.

4af3721

[giamir]

Work as advertised. Great work @dancormier

I am approving but let's remember we need to change stacks classic dep to 2.1.0 before merging and then cut the release.