feat: Implement on Document

[smockle]

Fixes #19

From that issue:

The ariaNotify explainer includes several examples of calling document.ariaNotify, so this should be supported by the polyfill. However, the polyfill only implements ariaNotify on Element (document instanceof Element is false).

This PR implements ariaNotify on Document.

[Copilot]

Pull Request Overview

This PR implements the ariaNotify method on the Document prototype to extend the ARIA Notification API polyfill beyond just Element. The implementation provides a way to send notifications at the document level.

• Added Document.prototype.ariaNotify method that delegates to documentElement
• Updated the polyfill condition to check for both Element and Document prototype support
• Updated README documentation to reflect the new Document.prototype.ariaNotify functionality

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
arianotify-polyfill.js	Implements Document.prototype.ariaNotify method and updates polyfill detection
README.md	Updates documentation to mention both Element and Document prototype methods

Comments suppressed due to low confidence (1)

arianotify-polyfill.js:178

• This condition is redundant since it's already checked at the top of the file (line 3). This nested check will always be true when reached, making the code unreachable if Element.prototype already has ariaNotify.

    ) {