Add FirstPopulated property (resolves #161)

* Add `FirstPopulated` property to return first populate page property

Author: CreggHancock

Source/ActivityPub.Types/AS/Collection/ASCollection.cs

@@ -112,8 +112,14 @@ public LinkableList<ASObject>? Items
     /// <summary>
     ///     True if this is a paged collection, false otherwise.
     /// </summary>
+    [MemberNotNullWhen(true, nameof(FirstPopulated))]
     public bool IsPaged => Current != null || First != null || Last != null;
 
+    /// <summary>
+    ///     In a paged Collection, gets the first populated entity.
+    /// </summary>
+    public Linkable<ASCollectionPage>? FirstPopulated => Current ?? First ?? Last;
+
     /// <summary>
     ///     True if this collection instance contains items, false otherwise.
     /// </summary>

Source/ActivityPub.Types/AS/Collection/ASOrderedCollection.cs

@@ -113,8 +113,14 @@ public LinkableList<ASObject>? Items
     /// <summary>
     ///     True if this is a paged collection, false otherwise.
     /// </summary>
+    [MemberNotNullWhen(true, nameof(FirstPopulated))]
     public bool IsPaged => Current != null || First != null || Last != null;
 
+    /// <summary>
+    ///     In a paged Collection, gets the first populated entity.
+    /// </summary>
+    public Linkable<ASOrderedCollectionPage>? FirstPopulated => Current ?? First ?? Last;
+
     /// <summary>
     ///     True if this collection instance contains items, false otherwise.
     /// </summary>