TypeScript typings: Separate writable computed.

I think it's better to separate the definitions for read-only computed and writable computed.
```
export interface ReadonlyComputed<T = any> extends ComputedFunctions<T> {
    (): T;
}

export interface WriteableComputed<T = any> extends ReadonlyComputed<T> {
    (): T;
    (value: T): this;
}
   
export interface ReadonlyPureComputed<T = any> extends ReadonlyComputed<T> { } 
```