fix(attributes): preserve non-boolean hidden attribute values

[rururux]

fixes: #5073

Changes

Previously, the .attr() method could not correctly retrieve the newly implemented until-found value for the hidden attribute. It would instead return "hidden" or undefined due to being treated strictly as a boolean attribute.
In this fix, I have removed hidden from the rboolean regex/variable to allow it to be handled as an enumerated attribute.

Testing

I have added a test case to ensure that the value of the hidden attribute can be correctly read from elements like <div hidden="until-found">.

[fb55]

Thanks @rururux! Could we still return a boolean if it is hidden, and extend the return type to "until-found"? Otherwise this will break old code.

[rururux]

@fb55
Thanks for the review! I’ve updated the code based on your feedback. Please take another look.