The shape of JSON schema snippets should be verified

An invalid JSON schema snippet is not reported back to the user.

Here is a minimal example which produces a confusing schema:

```
@implementation_specific
class Something_weird:
    pass
```

Snippet:
```
{"type": "number"}
```

We would expect the entry for "SomethingWeird" to be verbatim snippet with the definition name generated by aas-core-codegen.