[JS] Difficult to specify the result type on `Context`

When trying to type just the result of the backend request, you need to specify several irrelevant types which makes the resolver verbose and hard to grok at a glance. It's also very easy to forget which type parameter corresponds to what.

```ts
export function response(ctx: Context): string {
// ...
}
```

Start by locating the TypeScript definition of Context and the response resolver example shown in the issue. Check how its generic parameters are declared and used, then verify that a resolver can specify HttpResponse without supplying unrelated types. Done means the result type is expressible without the current sequence of never parameters, with relevant type-checking coverage added.

Written by the indexing model from the issue text.