[Question] why preffered_username field is not available on KeycloakTokenParsed type defination

[Manish Gowardipe]

Hi, 
we are using keycloakjs in our application in Typescript, I am looking for an answer on why `preferred_username` field is not made accessible to users or the reason behind why it is not present in typescript defination of `TokenParsed`.
if it is a miss, may i go ahead & raise a PR ? 
if it has been intentional kept private then will you recommend doing the following thing in any application which is using keycloak-js?
```

export interface Auth {

logout: (options?: KC.KeycloakLogoutOptions) => KC.KeycloakPromise<void, void>

token: () => string | undefined

tokenParsed: () => (KC.KeycloakTokenParsed & { preferred_username?: string }) | undefined

loadUserProfile: () => KC.KeycloakPromise<KC.KeycloakProfile, void>

isAuthenticated: () => boolean | undefined

}

```

Thanks