fix: Ensure proper closing of li element in plugin-editor.php

[debabratakarfa]

This commit ensures that the list item element in the plugin file tree navigation is closed adequately with a tag. The HTML structure in plugin-editor.php now correctly nests the tree structure with proper opening and closing tags.

This improves HTML validity and accessibility compliance for screen readers and other assistive technologies that rely on proper DOM structure, especially important for elements using ARIA roles like "treeitem" and "group".

Trac ticket: https://core.trac.wordpress.org/ticket/63546

Props dkarfa

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Props dkarfa.

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[westonruter]

While it makes sense to add this, it won't actually impact the structure of parsed HTML in the DOM. The parsing rules of HTML allow for implicitly-closed LI tags.

Compare the parsed DOM trees:

• Without the closing LI tag
• With the closing LI tag

The only difference is an extra whitespace #text node.

See MDN reference:

Tag omission: The end tag can be omitted if the list item is immediately followed by another <li> element, or if there is no more content in its parent element.

[joedolson]

Agreed. Also has no real world accessibility consequences, and doesn't violate WCAG as of version 2.2, when the relevant criteria was removed.

However, while no reputable accessibility company would consider it an issue, it is technically required under laws stipulating conformance with WCAG 2.0 or 2.1.

Still worth fixing. We just need to recognize that it's minor.

[debabratakarfa]

Noted. Thank you, @westonruter & @joedolson, for your feedback.