Add platform-provided behaviors for custom elements demo

[anaskim]

Adds an interactive demo page for platform-provided behaviors for custom elements. The demo showcases HTMLSubmitButtonBehavior, which allows a custom element to act as a native submit button through attachInternals({ behaviors: [...] }).

Use cases demonstrated

1. Basic form submission: a custom element submits a form the same way a native <button type="submit"> does.
2. Form override properties: setting name, value, and formMethod on the behavior instance, with a note on attributeChangedCallback for attribute reflection.
3. Implicit submission: pressing Enter in a text field triggers submission through the custom element.
4. Accessibility and keyboard activation: the element gets role="button", focusability, and Enter/Space activation with no extra code. Includes a comparison with what the developer would need to wire up manually.

Related links

• Explainer: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/PlatformProvidedBehaviors/explainer.md
• WHATWG issue: Platform-Provided Behaviors for Custom Elements whatwg/html#12150 (Platform-Provided Behaviors for Custom Elements whatwg/html#12150
• Spec PR: Add platform-provided behaviors for custom elements & submit behavior whatwg/html#12409 (Add platform-provided behaviors for custom elements & submit behavior whatwg/html#12409
• Chrome status entry: https://chromestatus.com/feature/6035800057839616