tBaseElement: Identity function for Privates

[danyill]

Currently our identity function doesn't process Private elements at all:

export function identity(e: Element | null): string | number {
  if (e === null) return NaN;
  if (e.closest("Private")) return NaN;
  const tag = e.tagName;

  if (isSCLTag(tag)) return tags[tag].identity(e);

  return NaN;
}

However we should use the mandatory type atttribute and index by this. This would be helpful with our diffing work for Private elements.

As discussed with @ca-d

[danyill]

For avoidance of doubt, the type attribute was optional in Ed. 1 and in Ed 2 it was mandatory.

SCTs are instructed in I.3.2.1 of IEC 61850-6 Ed 2.1 to add missing types with a value of "IEC61850_UpgradingRule" (which OpenSCD does not do at present but might be a sane default here).

[JakobVogelsang]

Nice this sound very very good

[ca-d]

For Private elements with a type attribute, taking the parent's identity and adding the type plus the position among Private elements of that type sounds perfectly viable, however in order to identify elements within a Private element, we will also need something even more general in addition, taking just the index of the node among elements with the same nodeName in addition to the parent's identity. That way, we will only return number if e === null. At that point we can add another function (e.g. nodeIdentity(node: Node): string) which does not take null as a parameter and in exchange always returns a string. That would be very useful to have, e.g. for serializing edits to the History.