Task: Remove System.Collections.Generic.ReadOnlyCollection<T> as a base class of ReadOnlyList<T>

[NightOwl888]

The J2N.ReadOnlyList<T> subclasses System.Collections.Generic.ReadOnlyCollection<T>. System.Collections.Generic.ReadOnlyCollection<T>.GetEnumerator() has changed in behavior in .NET 8 (or possibly .NET 7) so that it returns an enumerator that throws when it is empty. This means we have different behavior between .NET 8 and older targets. We had to add FEATURE_READONLYCOLLECTION_ENUMERATOR_EMPTY_CURRENT_UNDEFINEDOPERATION_DOESNOTTHROW as a workaround, since removing the subclass is a breaking change that requires a major version bump.

So, the task is to copy the contents of System.Collections.Generic.ReadOnlyCollection<T> to our ReadOnlyList<T> class (not sure why Microsoft screwed up the name, it accepts IList<T> rather than ICollection<T> through the constructor), remove the System.Collections.Generic.ReadOnlyCollection<T> base class, and remove the FEATURE_READONLYCOLLECTION_ENUMERATOR_EMPTY_CURRENT_UNDEFINEDOPERATION_DOESNOTTHROW so all of our tests are consistent.