Introduce `changesets` to our operations for better versioning

[shanefontaine]

I believe changesets is the industry standard for versioning, changelog, etc. It is recommended by pnpm. It is used by almost all projects.

Unknown in my head is how can this be done without adding a ton of unnecessary overhead for each version? It definitely makes sense for a large org like Optimism, but can we tailor it for speed with a smaller team?