Use assertion functions in our type definition

[novemberborn]

We should update our TypeScript definition so that our assertions are typed as such, see https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions.

That said, I haven't checked whether and how we can make this work for which assertions.

[novemberborn]

As per microsoft/TypeScript#34596 I don't think this will work for us. But I don't really understand any of the discussion. Let's see what happens in future TypeScript versions.

[sindresorhus]

Maybe relevant: microsoft/TypeScript#36067

[novemberborn]

Best I understand that issue, it'd still end up being useless for our use cases.