OPC-UA example from README not working

I tried to run the OPC-UA example `demo-opcua1.ts` from [README](https://github.com/eclipse-thingweb/node-wot/blob/master/packages/binding-opcua/README.md) (see [gist](https://gist.github.com/danielpeintner/e6c0f3723dce910f2dfcb8128f10b964) for full code).

When trying to run it I get the following error

```
Cannot figure out field category for { name: 'value', fieldType: 'Variant', defaultValue: null }
...\node_modules\node-opcua-factory\dist\builtin_types.js:262
throw new Error("Cannot find schema for simple type " + name);
```

The issue seems to come from [node-opcua](https://www.npmjs.com/package/node-opcua) and not from node-wot code.
@erossignon do you have any advice?

Start with demo-opcua1.ts and the binding-opcua README, then reproduce the reported error using the full code in the linked gist. Inspect the node-opcua stack trace and determine whether the example or its dependency setup is responsible. Done means the documented example runs without the schema error, or the external dependency limitation is clearly documented.

Written by the indexing model from the issue text.