Set up ESLint for the repo and fix lint issues

[tyom]

• Include ESLint 9 as root dependency
• Set up ESLint to lint the whole repo
• Extend the root config and add a few package-specific plugins for Evalite UI
• Add a consistent typecheck npm script for type checking across the repo

Use can now use pnpm lint in root and UI app and pnpm typecheck anywhere in the repo.
Use pnpm lint --fix to attempt to fix the issues.

Fix errors (mostly removing unused imports and variables). I left some prefixed with _ to serve as a reminder of function parameters.

Also add EditorConfig file to help maintain consistent code style for those who use it.

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Latest commit: 10ba908

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commit: 10ba908

[mattpocock]

@tyom There is a quite a lot of churn in the repo atm and most of our effort is focused on the v1 branch. Could you remake this PR to target that branch?