Given that we're (almost) always iterating through the result of the `getAll`-calls, using a `Map` seems nicer overall since it's more suited to iteration compared to a regular Object.

Also, add a couple of `Dict`-checks in existing code touched by this patch, since it really cannot hurt to prevent *potential* errors in a corrupt PDF document.

This patch first of all moves all checking/validation into the `appendIfJavaScriptDict` function, to avoid duplicating it in multiple places. Secondly, also removes what's now an outdated/incorrect comment since we have implemented scripting support.