Add option to prepend "readonly" to all fields

[Bluscream]

This would be nice to ensure i don't accidentally try playing with values from responses but create my own local copy instead

Example:

start "" json2ts -i "schemas/pietsmiet.de/*.schema.json" -o "src/responses/pietsmiet.de/" --inferStringEnumKeysFromValues true --readonly true

export interface PietsmietDEConfig {
    readonly API: Record<string, Record<string, string>>;
    readonly URLs: Record<string, string>;
    readonly Limits: Record<string, number>;
    readonly Orders: Record<string, string>;
    readonly Platform: Record<string, string>;
    readonly Stream: Record<string, string>;
  }
...

[bcherny]

It would make sense to emit all types as readonly, gated behind a --readonly option.