const and readonly should be converted to val

```typescript
export declare const SomeConst: string;
export interface InterfaceA {
readonly readOnlyProp: boolean;
}
```
```kotlin
external var SomeConst: String = definedExternally // <-- should be val
external interface InterfaceA {
var readOnlyProp: Boolean // <------------------------should be val
}
```

Start by tracing Dukat's TypeScript-to-Kotlin declaration conversion for const declarations and readonly properties. Reproduce the example from the issue, then verify that SomeConst and readOnlyProp are emitted as val rather than var.

Written by the indexing model from the issue text.