add generic support on RegisterCustomType()

[enginekit]

add generic support on RegisterCustomType()

      JSON.RegisterCustomType<Point, List<object>>(
         //since we know that our point will be serialized to list of objects
         pt => string.Format("[{0},{1}]", pt.x, pt.y),
         (l, cb) =>
         {
             return new Point() { x = Convert.ToInt32(l[0]), y = Convert.ToSingle(l[1]) };
         });
    //
    JSON.RegisterCustomType<Complex, List<object>>(
        //since we know that our complex type will be serialized to list of objects
        c => string.Format("[{0},{1}]", c.i, JSON.ToJSON(c.l)),
        (l, cb) =>
        {
            return new Complex() { i = Convert.ToInt32(l[0]), l = (List<Point[]>)cb(l[1], typeof(List<Point[]>)) };
        });

https://github.com/enginekit/fastJSON/blob/f100fcd5b188877351fbb798ca9a73fa01d69edd/UnitTests/Tests.cs#L1939