edit/revertCell (#268)

// edit/dispose -------------------------------------------------------------------------------
export interface EditDisposeParams extends IEditSessionOperationParams { }
export interface EditDisposeResult { }

* Initial plumbing for edit/revertCell

* Implementation of revert cell in the parents of the row edit base

* Adding unit tests

test/Microsoft.SqlTools.ServiceLayer.UnitTests/EditData/RowEditBaseTests.cs

@@ -267,6 +267,11 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.EditData
                 throw new NotImplementedException();
             }
 
+            public override string RevertCell(int columnId)
+            {
+                throw new NotImplementedException();
+            }
+
             public override EditUpdateCellResult SetCell(int columnId, string newValue)
             {
                 throw new NotImplementedException();