Allow the visitor to cease callbacks

[Vbbab]

As per #30, this PR implements a new feature whereby the callbacks for onObjectBegin and onArrayBegin for the JSONVisitor may return a boolean (rather than an arbitrary ignored value as was previously the case) which indicates whether the visitor should continue receiving callbacks for the subtree.

As an example, consider the following JSON:

{
    "a": {
        "b": [ {
            "c": "d",
            "e": "f",
        } ]
    }
}

If, upon receiving the onArrayBegin callback from the array of b, the user returns false from the callback, then all other callbacks for everything within the array will be suppressed, and the next callback the user receives will only be the corresponding onArrayEnd.

Internally, this works by keeping a counter suppressedCallbacks within visit(), which keeps track of the "depth" of the subtree where callbacks are being suppressed. When a new onXXXBegin() callback is called and this depth is nonzero, it is incremented. Similarly, when an onXXXEnd() callback is called, the depth is decremented, and callbacks are only called when the counter is 0. This also meant creating new callback adapters (?) such as toBeginVisit and toEndVisit that handle this counter logic on top of wrapping the actual callback. These were modeled after the original toNoArgVisitWithPath/toNoArgVisit functions.

FWIW, I've changed the return type of the two begin callbacks to boolean | void rather than just boolean, so as to preserve some backward compatibility for users who weren't just returning garbage from the callbacks. However, this also meant some refactoring to other parts of the codebase that didn't quite comply with a strict void return.

I've also added additional parameters to the assertVisit function to test stopping callbacks at a certain position within the JSON (as long as an onArrayBegin or onObjectBegin callback can be triggered at the specified position). In order to adapt the fact that the begin callbacks now return booleans, I added a beginHalder which handles the logic of returning false when appropriate.

I'd appreciate it if you would be kind enough to review my code. Let me know what you think!
Thank you!