Added Range Manipulation APIs to Collection<T> and ObservableCollection<T>

[SkyeHoefling]

Added Range Manipulation APIs to Collection and ObservableCollection.

Part of : dotnet/corefx#10752 - migrated to: dotnet/runtime#18087
Part of: dotnet/corefx#35772

Added the following new API that was approved:

    // Adds a range to the end of the collection.
    // Raises CollectionChanged (NotifyCollectionChangedAction.Add)
    public void AddRange(IEnumerable<T> collection) => InsertItemsRange(0, collection);

    // Inserts a range
    // Raises CollectionChanged (NotifyCollectionChangedAction.Add)
    public void InsertRange(int index, IEnumerable<T> collection) => InsertItemsRange(index, collection);

    // Removes a range.
    // Raises CollectionChanged (NotifyCollectionChangedAction.Remove)
    public void RemoveRange(int index, int count) => RemoveItemsRange(index, count);

    // Will allow to replace a range with fewer, equal, or more items.
    // Raises CollectionChanged (NotifyCollectionChangedAction.Replace)
    public void ReplaceRange(int index, int count, IEnumerable<T> collection)
    {
         RemoveItemsRange(index, count);
         InsertItemsRange(index, collection);
    }

    #region virtual methods
    protected virtual void InsertItemsRange(int index, IEnumerable<T> collection);
    protected virtual void RemoveItemsRange(int index, int count);
    #endregion

Looking forward to everyone's feedback on this

[ahsonkhan]

Based on https://github.com/dotnet/corefx/issues/10752#issuecomment-428312248, you are missing ReplaceItemsRange

[SkyeHoefling]

Based on dotnet/corefx#10752 (comment), you are missing ReplaceItemsRange

Thanks for pointing this out, I was going off the update at the top and missed this API. I'll get this added in the next update and include tests in the associated PR for CoreFX

[ahsonkhan]

Some leftover comments. Otherwise, LGTM.

[safern]

LGTM once @ahsonkhan's last comments are solved.

[safern]

Thanks @ahoefling now we can move to corefx's PR once CoreCLR flows there 😄

[jkotas]

@safern Try to hit squash & merge next time ... I know it is easy to press the wrong button by accident.

[safern]

@safern Try to hit squash & merge next time ... I know it is easy to press the wrong button by accident.

Yeah I just realized I rebased and merge. Sorry about that, my default was "Squash and merge" I don't know how it was changed 😭