various dependency updates

[bkendall]

• drops node 18
• upgrades eslint
• upgrades typescript

[Copilot]

Pull Request Overview

This PR updates various dependencies and removes Node.js 18 support. It primarily involves upgrading ESLint, TypeScript, and related tooling while maintaining compatibility with Node.js 20, 22, and 24.

• Drops Node.js 18 support from engines specification and CI matrix
• Upgrades ESLint from 9.19.0 to 9.35.0 and related plugins
• Upgrades TypeScript from 5.3.0 to 5.9.2 and typescript-eslint from 8.23.0 to 8.43.0

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
package.json	Updates engine constraints, ESLint, TypeScript, and related dev dependencies
eslint.config.mjs	Adds new TypeScript ESLint rule for nullish coalescing
changelog.txt	Documents Node.js 18 removal
.github/workflows/node-test.yml	Removes Node.js 18 from CI test matrix

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