ClassDef in state diagrams not supported

[Chadehoc]

Expected behaviour

With some italics:

```mermaid
stateDiagram-v2
    a: blabla
    b: hello
    a --> b

    classDef em font-style:italic
    class a em
```

stateDiagram-v2
    a: blabla
    b: hello
    a --> b

    classDef em font-style:italic
    class a em

Loading

Actual behaviour

```mermaid
diagram is not syntactically correct
```