fix: [#1888] Replaces inefficient loops for clearing children

[TrevorBurnham]

Fixes #1888

This PR introduces a fast path for clearing all children from a parent node in O(n) time and wires it into hot code paths (innerHTML, textContent, replaceChildren, ShadowRoot.setHTMLUnsafe, and Document.open). It also optimizes cleanup in Node#[PropertySymbol.cloneNode] using the same technique. The change preserves observable behavior and MutationObserver semantics while significantly reducing time and allocations for large subtree clears.

Motivation

Several core DOM APIs in happy-dom clear a node by repeatedly calling removeChild(childNodes[0]) in a loop. Each removal from index 0 shifts the array, leading to O(n^2) behavior and extra GC pressure for large trees. This PR replaces those loops with a single-pass algorithm.

Design overview

• New utility: ParentNodeUtility.clearChildren(parent)

  • Detaches and cleans all children in a single sweep.
  • Preserves NodeList reference identity by truncating arrays (length = 0) rather than reassigning them.
  • Clears elementArray in one step.
  • Reproduces the same sequence of childList MutationRecords that the previous loops emitted, matching previousSibling and nextSibling expectations.
  • Ensures correct lifecycle transitions (connected/disconnected) and cache invalidations.
  • Unobserves subtree mutation observers from removed subtrees as before.

• Node#[PropertySymbol.cloneNode] cleanup

  • Replaces the previous while (childNodes.length) removeChild(childNodes[0]) pattern with a local fast-detach that snapshots children, truncates arrays (length = 0), and then clears parent links/cache and dispatches disconnects per child.