Replace any with unknown/never where needed

`any` is a type that should only be used in places where we want to turn off strict typing. In most cases, you want to either use `never` as a bottom type or `type unknown = {} | null | undefined` as a top type. I'm not sure yet whether to use an alias (as every other library also defines that same alias, or just use the longer union of three types, but be more clear about what we're doing.