Mark class [Serializable] to address warning

Addresses warning CA2237: https://docs.microsoft.com/en-us/visualstudio/code-quality/ca2237-mark-iserializable-types-with-serializableattribute

LiteDB.Shell/Shell/ShellException.cs

@@ -2,6 +2,7 @@
 
 namespace LiteDB.Shell
 {
+    [Serializable]
     internal class ShellException : Exception
     {
         public ShellException(string message)
@@ -14,4 +15,4 @@ public static ShellException NoDatabase()
             return new ShellException("No open database");
         }
     }
-}
+}