Add react-dom as a peer dependency

[thebuilder]

Adds react-dom as a peer dependency to the project.

• It's not needed by the intersection library itself, but the test tooling requires the act export.
• You can only use react-intersection-observer in the browser, so you'd need it anyway.
• Mark it as optional, just to be sure it doesn't break anything: https://docs.npmjs.com/cli/v7/configuring-npm/package-json#peerdependenciesmeta

Closes #657

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Name	Status	Preview	Comments	Updated (UTC)
react-intersection-observer	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 31, 2024 9:53pm

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[thebuilder]

🎉 This PR is included in version 9.6.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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