Xml Default Namespace

As part of xml basic design #2970 we don't support default `xmlns`. This can be quite problematic so moving into a different issue

```xml

abc
abc

```

Here:

- `Book` belongs to `https://example.com/ns1`
- `id` belongs to `https://example.com/ns2`
- `name` belongs to `https://example.com/ns1`

```tsp

@defaultNamespace("https://geo-json.com")
namespace GeoJson {
model Point {
x: string;
y: string;
}
}

@defaultNamespace("https://my-service.com")
namespace MyService {
model Foo {
loc: GeoJson.Point;
myLoc: MyPoint;
}

model MyPoint {
...GeoJson.Point;
}
}
```

```xml

1
1


1
1

```