Add `ReflectionProperty::isDynamic()` as an alternative to `isDefault()` (#15758)

Dynamic properties are generally referred to as "dynamic" properties, while
non-dynamic properties are n

Add `ReflectionProperty::isDynamic()` as an alternative to `isDefault()` (#15758)

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

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