Split typechecking from tests

[NullVoxPopuli]

This allows us to more fine-grainedly seeing what's going on in CI, without just blanked "red" vs "green" status.

This does cost more install time -- but if we eventually move to pnpm, we'll re-gain that time back.

tl;dr:

• pull out linting to its own job
• pull out type checking to its own job
• fix linting for the unstable-release package
• resolve lint errors in the rest of the repo
• upgrade to prettier 3
• lint:fix (most of the changes)

probably best to review per-commit

Ran in to the need for yarnpkg/yarn#6739
but ugh.