Update dependencies and lint

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Problem

• stale dependencies
• using deprecated tseslint.config, and eslint now has defineConfig()

Solution

Update dependencies. Fix new lint errors. Switch from tseslint.config to defineConfig()

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One of the features of the config helper is it handles whether the config is an object or an array of objects, and likewise for the extends, so removed the spread operator.

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For interest, typescript-eslint 8 supports eslint 8, 9, 10. Its version is not locked to the eslint version number.

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I could perhaps have done this on develop, but now focused on 15.x and made the changes on that branch.