[proposal] extend hx-preserve functionality to optionally preserve attibutes and classes only

hx-preserve currently only supports the preservation of the whole element which means that it can not be used if we want to update the data. We can however extend its functionality by accepting parameters in a following manner:
`hx-preserve="attr:open"` and `hx-preserve="class:hidden" ` This way we can preserve specific attributes or classes.

implementation seems quite simple:
```
` function handlePreservedElements(fragment) {
forEach(findAll(fragment, '[hx-preserve], [data-hx-preserve]'), function (preservedElt) {
var id = getAttributeValue(preservedElt, "id");
var oldElt = getDocument().getElementById(id);
if (oldElt == null){
return
}
var parameters = getAttributeValue(preservedElt, 'hx-preserve')
if (parameters){
preserveAttributesAndClasses(oldElt, preservedElt, parameters)
} else {
preservedElt.parentNode.replaceChild(oldElt, preservedElt);
}
});
}

function preserveAttributesAndClasses(oldElt, preservedElt, parameters){
forEach(parameters.split(","), function (parameter){
{
var splitted_parameter = parameter.split(":")
var qualifier = splitted_parameter[0]
var name = splitted_parameter[1]
if (qualifier === 'attr') {
if (oldElt.hasAttribute(name)) {
preservedElt.setAttribute(name, oldElt.getAttribute(name))
} else {
preservedElt.removeAttribute(name)
}
} else if (qualifier === 'class') {
if (oldElt.classList.contains(name)) {
preservedElt.classList.add(name)
} else {
preservedElt.classList.remove(name)
}
}
}
})
}
```