JsonSchema shouldn't union of primitive types use multiple type instead of `anyOf`

```tsp
model Foo {
optional?: string | null = null;
}
```

currently produce

```yaml
$schema: https://json-schema.org/draft/2020-12/schema
$id: Foo.yaml
type: object
properties:
optional:
anyOf:
- type: string
- type: "null"

```

but shouldn't it make

```yaml
$schema: https://json-schema.org/draft/2020-12/schema
$id: Foo.yaml
type: object
properties:
optional:
type:
- string
- "null"

```

[Playground Link](https://cadlplayground.z22.web.core.windows.net/?c=aW1wb3J0ICJAdHlwZXNwZWMvanNvbi1zY2hlbWEiOwoKdXNpbmcgVHlwZVNwZWMuSnNvblPFHTsKCkDELMYOCm5hbWVzcGFjZSDGEXM7Cgptb2RlbCBGb28gewogIG9wdGlvbmFsPzogc3RyxFZ8IG51bGwgPcUHOwp9Cg%3D%3D&e=%40typespec%2Fjson-schema&options=%7B%7D)