[Bug]: EmitterOptions defaults are not resolved.

### Describe the bug

Following the same example of custom configs definitions from here: https://typespec.io/docs/extending-typespec/emitters-basics/#custom-configuration-options, but using default for some property, doesn't in runtime doesn't result in default value being passed into emitter, so the typing is off.

### Reproduction

E.g. having this from documentation:

```typescript
export interface EmitterOptions {
"target-name": string;
}

const EmitterOptionsSchema: JSONSchemaType = {
type: "object",
additionalProperties: false,
properties: {
"target-name": { type: "string", nullable: true, default: null },
},
required: [],
};
```

In emitter

```typescript
export const $lib = createTypeSpecLibrary({
internal: internalLib,
emitter: {
options: EmitterOptionsSchema,
},
});

export async function $onEmit(context: EmitContext) {
console.log(contenxt.options); // --> {}
}
```

If I'm providing json-schema, I'd expect defaults set in it to be used. Otherwise it doesn't make a lot of sense and usage of json-schema in this case should be documented on the terms of scopes it's being applied for. R.n. seems like only for IDE autocomplete integration for emitters, but no types validation nor defaults substitution.

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.