Add ::selection to root color vars

[langermank]

Fixes: https://github.com/github/github/issues/264779

What are you trying to accomplish?

We're getting several bug reports in Chrome related to color vars and ::selection due to a Chromium experiment.

• Highlight color is muted #2411

What approach did you choose and why?

Based on this article, maybe adding ::selection will fix the problem?

What should reviewers focus on?

Will this break the world?

Can these changes ship as is?

• Yes, this PR does not depend on additional changes. 🚢

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🦋 Changeset detected

Latest commit: cea2c16

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/css	Patch

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[keithamus]

@langermank This PR will need reverting because the introduction of ::selection blows the whole selector away. The ::selection style needs to be standalone it seems. In particular, the following:

@include color-mode("light") {
  p {
    color: green
  }
}

gets compiled into

:root p,
::selection p {
  color: green
}

which it seems is discarded. Here's a minimal repro to demonstrate that the selector is discarded:

https://codepen.io/keithamus/pen/mdQrGMo

Refs https://github.com/github/issues/issues/6741, https://github.com/orgs/community/discussions/58723#discussioncomment-6250327