No support for `addV` only `addVertex`

[kuzeko]

As per Gremlin documentation:

The Graph API (also referred to as the Structure API) is not always accessible to users. Its accessibility is dependent on the choice of graph system and programming language. It is therefore recommended that users avoid usage of methods like Graph.addVertex() or Vertex.properties() and instead prefer use of Gremlin with g.addV() or g.V(1).properties().

https://tinkerpop.apache.org/docs/current/reference/#staying-agnostic

Currently, using gts.addV generates an error.