chore(ci): type-check library in CI

[KnorpelSenf]

How did we never type-check grammY in CI? It currently fails and I only discovered this locally.

[winstxnhdw]

Do we have to add /// <reference types="npm:@types/node" /> in here as well? I always assumed deno task test also type-checks grammY, or at least it seems like it does since it fails whenever I make any type errors that are also not runtime errors.

[KnorpelSenf]

I assumed the same thing. It might only check test files, not all modules under test.

I don't think we can use triple-slash directives here because that would pollute the global types at runtime. We have to find a different way.

[winstxnhdw]

Ok, let me handle this.