sanitization: add support for HTML `hidden` attribute

[JohnnyJayJay]

This adjusts the Sanitize function to skip tags with the hidden attribute, similar to how it skips blocked tags and their contents.

From the whatwg HTML spec:

User agents should not render elements that are in the hidden state.

An alternative implementation would be to always propagate the hidden attribute to the sanitised HTML content, i.e., let the browser deal with hiding it.

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• [x] I have tested my changes
• [x] There is no breaking changes
• [x] I really tested my changes and there is no regression
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[JohnnyJayJay]

I ran gofmt -s -w on the problematic file, should be good now.