Add Turborepo

[domdomegg]

Improve a lot of our setup, including:

• Add Turborepo, to better manage our monorepo setup. This isn't fully optimized yet: I think there are definitely places where we could improve the explicitness of task dependencies and move them into Turborepo from the NPM scripts. We could also benefit from standardizing the input and output directories further.
• Add cspell configuration, including recommending the extension. I struggle with spelling, and this helps keep things neat and easily-understandable (including to GPT-like / translation / other tools). I've also gone through and fixed a bunch of the spelling mistakes it found. I'm tempted to make this part of the CI check, but I suspect this will be more annoying than helpful.
• Moved around a lot of the docs, to reduce duplication between packages and make it easier for people to discover the docs relevant to them. Also added documentation on using Turborepo.
• Standardized ESLint configs (mostly), and fixed lint issues
• Ensured everything works smoothly in VS Code (e.g. TypeScript, ESLint)
• Updated CI to use NPM caching + Turborepo + Turborepo caching, so it completes faster

Things still to improve

• DynamoDB local: AWS seem to like making unannounced breaking changes, plus upstream doesn't seem to maintain things. This has been the cause of multiple unexpected breakages over the last few months, and I have come to the point where I think it's worth making our own stable fork that protects us from this somewhat.
• [for later] Formatting: it would be good to get prettier set up and working
• [for later] MWA build: it would be nice to make this less hacky
• [for later] Tests: we might migrate to Vitest, given it seems less prone to weird babel problems and is faster, plus the migration path is pretty easy.