chore: create separate CI jobs for linting and testing

[achou11]

Removes the linting step done in the npm test script. Instead, we run a separate CI job that handles just the linting checks so that it doesn't potentially interfere with tests running.

So now we have two jobs that run in our CI workflow:

1. Lint - runs prettier + eslint checks
2. Test - runs TypeScript checks + Vitest tests

Could potentially argue that TypeScript checks should live in (1), but I think it's fine to keep in (2) for now, which aligns with how we have other repos set up.