Fix push() dropping valid items after an invalid/fctn entry, and remo…

[kenmckell]

This PR fixes two related bugs in push(), both reproducible on unmodified master.

1. Multi-item push() calls could silently drop valid items.

push() tracked which items should end up in the array by deleting invalid/function/listener entries out of the same array holding the original arguments, then only checked whether index 0 of that array survived before deciding whether to add anything at all:

if (filteredArguments[0]) {
  return Array.prototype.push.apply(this, filteredArguments);
}

Since delete leaves a hole rather than shifting later items down, if the first argument in a call was invalid or a function, every other valid item in that same call was silently excluded from the array — even though its data was still merged into state and listeners still fired correctly.

2. Pushing a listener object directly ({ on: ..., handler: ... }) registered nothing.

The LISTENER_ON/LISTENER_OFF case only removed the item from the array-bound arguments; it never called _processItem(), so registration/removal never actually happened. Only addEventListener()/removeEventListener() worked.

Fix: push() now builds a clean, independent list of items that should be visible, and calls _processItem() for every recognized item type, including listeners. Also removed dead code in cloneDeepWith (src/utils/mergeWith.js) that built a manual recursive clone which was always immediately discarded in favor of structuredClone.

Testing: I wasn't able to get the full local Jest suite to run cleanly in my environment — events.test.js and several other suites fail with RangeError: Maximum call stack size exceeded inside @ungap/structured-clone. I confirmed this is pre-existing and unrelated to this change (reproducible identically on unmodified master). Per that polyfill's own documentation, this specific error occurs when it's attached to the global scope without checking for a native structuredClone implementation first — likely a version mismatch between an older pinned Jest dependency and newer Node versions (tested on Node v22.22.1) that already provide structuredClone natively.