chore: add engines field to specify Node.js version requirement

[arpit-jn]

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Description

This PR adds the engines field to package.json to explicitly specify the minimum Node.js version requirement.

Problem:

• Developers using Node.js versions below 18.18.0 encounter confusing EBADENGINE warnings from @typescript-eslint/* packages without a clear indication of the root cause
• The CI pipeline uses Node 18, but this requirement was not documented, leading to "works in CI but not locally" issues

Solution:

"engines": {
  "node": ">=18.18.0"
}

Why 18.18.0?

• Aligns with @typescript-eslint/* packages which require ^18.18.0 || ^20.9.0 || >=21.1.0
• Matches the Node version used in CI workflows (node-version: 18)

Impact:

• Non-breaking: The engines field only produces warnings by default, not hard failures
• Users can still install with --ignore-engines if needed
• Improves developer experience by making implicit requirements explicit

References

• Node.js 18.18.0 release: https://nodejs.org/en/blog/release/v18.18.0

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not the default branch