Dynamic properties are generally referred to as "dynamic" properties, while
non-dynamic properties are not commonly referred to as "default" properties.
Thus, the existing method `ReflectionProperty::isDefault()` has a non obvious
name; while an alias could be added for `isNotDynamic()`, a new `isDynamic()`
method seems cleaner. The new method returns the opposite of `isDefault()`;
dynamic properties are not present on the class by default, and properties
present by default are not added dynamically.
Closes GH-15754