chore: upgrade to eslint 9.x

[43081j]

Upgrades ESLint and moves to the new flat config style.

Also fixes up some newly found lint errors/warnings and enables TSESLint's strict config.

I was roaming around the repo so figured I'd do this while I'm in there. if you don't want it, all good just let me know and we can close this 👍

Notes

• Locally, for whatever reason, npm run build:ts fails with a lot of errors. is that known to be broken? (in main)
• Do we need our own full blown config still? maybe we could use the recommended configs from ESLint, stylistic and tseslint
• could we introduce a formatter like prettier or dprint, and drop stylistic?

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