Added WeakRefMessenger type

Author: Sergio0694

Microsoft.Toolkit.Mvvm/Messaging/Microsoft.Collections.Extensions/DictionarySlim{TKey,TValue}.cs

@@ -130,7 +130,7 @@ public void Clear()
             this.entries = InitialEntries;
         }
 
-        /// <inheritdoc cref="Dictionary{TKey,TValue}.ContainsKey"/>
+        /// <inheritdoc/>
         public bool ContainsKey(TKey key)
         {
             Entry[] entries = this.entries;

Microsoft.Toolkit.Mvvm/Messaging/Microsoft.Collections.Extensions/IDictionarySlim{TKey}.cs

@@ -13,12 +13,19 @@ namespace Microsoft.Collections.Extensions
     internal interface IDictionarySlim<in TKey> : IDictionarySlim
         where TKey : IEquatable<TKey>
     {
+        /// <summary>
+        /// Checks whether or not the dictionary contains a pair with a specified key.
+        /// </summary>
+        /// <param name="key">The key to look for.</param>
+        /// <returns>Whether or not the key was present in the dictionary.</returns>
+        public bool ContainsKey(TKey key);
+
         /// <summary>
         /// Tries to remove a value with a specified key.
         /// </summary>
         /// <param name="key">The key of the value to remove.</param>
         /// <param name="result">The removed value, if it was present.</param>
-        /// <returns>.Whether or not the key was present.</returns>
+        /// <returns>Whether or not the key was present.</returns>
         bool TryRemove(TKey key, out object? result);
 
         /// <summary>