Add ModelProperty.isId

Add `ModelProperty.isId` that is serialized as `id: bool` in `models.json`:

```js
{
  "name": "Car",
  "properties": {
    "id": {
      "type": "string",
      "id": true, // <-- added by this patch
      "generated": true
   }
}
```

Implement support for LDL annotation allowing entity properties to specify a different name to use when serializing to config JSON.

/to @ritch please review