Suggest using `hash` instead of `make-immutable-hash` with quasiquoting

Example:
```scheme
(make-immutable-hash
 `((body . ,body)
   (event . ,event)
   (comments . ,(map github-review-comment-jsexpr comments))))
```
If the keys are statically known, `hash` is simpler.

The same goes for quasiquoted `#hash()` literals, but those are trickier.