Add ToElementOrDefault<T>, which returns null for an element of another type #121

Description of the new feature / enhancement

ToElementOrDefault<T> returns the element of the type T with an id, or null for an id of no element or of an element of another type.

ToElement<T> throws InvalidCastException for an element of another type, the way LINQ First throws for an empty sequence.
ToElementOrDefault<T> follows the convention of FirstOrDefault:

var view = elementId.ToElementOrDefault<View>(document);
if (view is null)
{
    return;
}

In the preceding code, view is null when elementId names a wall, and when it names no element of the document.

• ToElement<T> throws for an element of another type, as it does now.
• ToElementOrDefault<T> sits beside ToElement<T> in the element id extensions, with the same constraint where T : Element.

Scenario when this would be used?

A tool resolves an id it receives from outside, such as an id a user or another process passes, and treats an id of another element as no match.
Today it writes document.GetElement(elementId) as View in place of the extension.

The agent skill that recommends the extensions changes with it: Nice3point/revit-skills#17.