Allow implementation of delete operation in foreign arrays

[lucas-mpc-brant]

As discussed here currently it is not possible to delete elements in non-javascript arrays. I'd like to ask for the ability to include our own implementation of what a delete operation should do.

Code sample that throws the related error:

final List<Object> myList = new ArrayList<>();
for (int i = 0; i < 4; i++) {
    myList.add(i);
}

final ProxyArray arr = ProxyArray.fromList(myList);
final Context context = Context.newBuilder("js").option("js.strict", "true").build();

context.getBindings("js").putMember("myArr", arr);
context.eval("js", "delete myArr[2]");
context.eval("js", "myArr[3] = 10");

final Value aux = context.eval("js", "myArr");

[iamstolis]

JavaScript has a bit unusual semantics when it comes to the deletion of array elements: it removes the element and leaves a hole there. Other languages typically do not support deletion at all or shrink the array (i.e. move the elements to fill the hole and decrease the size of the array/list). That's why we hesitate to implement the general delegation of JavaScript delete operation to a custom/non-javascript remove[ArrayElement]().

A possible workaround is to wrap the non-javascript array in JavaScript proxy, i.e., create JS Proxy whose target is the array and provide a custom deleteProperty trap in the handler of this proxy. Unfortunately, it turned out that there are several bugs that are blocking this approach. I have fixed these bugs and created a simple example of the mentioned approach.