Made MudSelectExtended IsOpen flag public (#469)

* Made MudSelectExtended IsOpen flag public

* Reverted changes to isOpen, making it internal again and isntead added a public method to return it's value

* Removed xml summary as field is no longer public

Author: m-flex

CodeBeam.MudBlazor.Extensions/Components/SelectExtended/MudSelectExtended.razor.cs

@@ -1305,5 +1305,14 @@ protected async Task ChipClosed(MudChip<T>? chip)
             SelectedValues = SelectedValues.Where(x => x?.Equals(chip.Value) == false);
             await SelectedValuesChanged.InvokeAsync(SelectedValues);
         }
+        
+        /// <summary>
+        /// returns the value of the internal property _isOpen 
+        /// </summary>
+        /// <returns></returns>
+        public bool GetOpenState()
+        {
+            return _isOpen;
+        }
     }
 }