refactor: add return early to useMergedRefs

[joshblack]

Add support for detecting the major version for React so that we can easily toggle on functionality supported in React 19 vs 18.

Changelog

New

Changed

• Update environment.js to environment.ts
• Update useMergedRefs to return early if React <19 (helps to prevent console warnings in React 18 for clean up functions)

Removed

Rollout strategy

• Patch release

[changeset-bot]

🦋 Changeset detected

Latest commit: 0adf41d

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

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions]

⚠️ Action required

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[Copilot]

Pull request overview

Converts environment.js to TypeScript, adds reactMajorVersion detection, and short-circuits the useMergedRefs cleanup function on React <19 to suppress the React 18 warning about cleanup functions being ignored.

Changes:

• Rename utils/environment.js → utils/environment.ts and export reactMajorVersion parsed from React's version string.
• In useMergedRefs, return early before the cleanup function when running on React <19.
• Add patch-level changeset.

Show a summary per file

File	Description
packages/react/src/utils/environment.ts	New TS module exporting canUseDOM and reactMajorVersion.
packages/react/src/utils/environment.js	Old JS file removed (replaced by .ts).
packages/react/src/hooks/useMergedRefs.ts	Skips returning the ref cleanup function on React <19.
.changeset/dull-berries-call.md	Patch changeset documenting the React 18 warning fix.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 0

[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.