Can't easily initialise classes in Web Investigator REPL

Currently this happens:

```javascript
class X {}
// <- undefined
new X();
// ReferenceError: X is not defined
```

Right now, you instead have to do this:

```javascript
var X = class {};
```

We need to allow classes to be instantiated using the non-assignment syntax like we have allowed for functions. We also need to ensure that this works when extending classes.