Prepare 18.0.0

write-file-atomic is our first dependency to drop support for Node.js 20:

• https://github.com/stylelint/stylelint/pull/9303

So now feels like a good time to open an issue to prepare for our next major.

Let's think about the breaking changes we'd like to make, keeping in mind that we try to walk a fine line between modernising and maintaining compatibility.

Once we have a sense of the task ahead of us, we can decide when we want to start working on it.

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Reminder to mention referenceFiles in the migration guide for visibility.

[!IMPORTANT] Remember to:

• update repo ruleset when closing this https://github.com/stylelint/stylelint/issues/9338#issuecomment-5586062520
• switch dependabot target branch