fix: make isElement compatible with iframe elements

[LynnDolly]

Summary

instanceof Element can fail for elements created in a different browsing context (e.g. iframe),
because each window has its own Element constructor.

This change updates isElement to support cross-iframe elements by:

1. keeping the fast path for current-window elements:
   • node instanceof Element
2. adding a fallback path:
   • resolve node.ownerDocument.defaultView.Element
   • check node instanceof ElementConstructor

Why

element-source may receive nodes from embedded documents.
Without this fallback, valid iframe elements may be treated as non-elements.

Scope

• packages/element-source/src/utils/is-element.ts
• no public API changes
• behavior only improves for cross-window/iframe cases

[vercel]

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