HTML API: Consistent case sensitive handling of class names

[sirreal]

Trac ticket: https://core.trac.wordpress.org/ticket/61531

The Tag Processor CSS class name methods class_list, add_class, and remove_class should be consistent regarding case sensitivity.

These methods are intended to align with CSS class names, meaning that matching should be done ASCII case-insensitive. class_list already yields lower case unique class names, but remove_class and add_class do not have similar behavior of treating case-insensitive matching classes as equal.

• add_class should only add classes that are not already present (compared ASCII case-insensitive).
• remove_class should remove all matching classes (compared ASCII case-insensitive).

This was discussed with @dmsnell on Slack here: https://wordpress.slack.com/archives/C05NFB818PQ/p1719403633636769
Related to #61520 which documents the lower-casing behavior of class_list.

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[github-actions]

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[sirreal]

We need to be able to match the class_name case insensitively, but still add whatever class was provided.

I changed the values to be an array of one of the following shapes:

• [ self::ADD_CLASS, (string) $class_to_add ]
• [ self::REMOVE_CLASS ]
• [ self::SKIP_CLASS ]

[sirreal]

This doesn't seem like the right approach, it may not align with browser behavior.