fix: make open property of VtmnAccordion reactive

[Martin-Brandenburg]

Changes description

This is a bugfix to the VtmnAccordion component. The open prop was binded in the details element.

Context

The previous implementation only allows us to pass the open prop down, but changes are not reflected. In particular, we cannot let the parent of the VtmnAccordion know when it is open and when not. So this is just a missing feature, but: there is also a bug in the previous implementation: the line aria-expanded={open} means that the aria-expanded property is always set to the initial value. It will never be changed.

When the accordion is closed, it does not have the open prop and we have aria-expanded=false, which is correct:

But when it is opened, the open prop is added, but aria-expanded=false stays.

This pull request fixes this bug.

Checklist

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Does this introduce a breaking change?

• No

[CLAassistant]

All committers have signed the CLA.

[lauthieb]

Thanks @Martin-Brandenburg for this proposition!
I'll let @thibault-mahe review it. @Tlahey feel free to check it out also if you can, please 🙏.
@Martin-Brandenburg can you please sign the CLA? #1461 (comment)