Json Schema should a union of literals be converted to an enum

```
model Foo {
a: "one" | "two"
}
```

```yaml
$schema: https://json-schema.org/draft/2020-12/schema
$id: Foo.yaml
type: object
properties:
a:
anyOf:
- type: string
const: one
- type: string
const: two
required:
- a
```

openapi3
```yaml
Foo:
type: object
required:
- a
properties:
a:
type: string
enum:
- one
- two
```
[Playground Link](https://cadlplayground.z22.web.core.windows.net/?c=aW1wb3J0ICJAdHlwZXNwZWMvanNvbi1zY2hlbWEiOwoKdXNpbmcgVHlwZVNwZWMuSnNvblPFHTsKCkDELMYOCm5hbWVzcGFjZSDGEXM7Cgptb2RlbCBGb28gewogIGE6ICJvbmUiIHwgInR3byIKfQ%3D%3D&e=%40typespec%2Fjson-schema&options=%7B%7D)