Better initiate `createOutputScreen()`

Right now we have to have all constants come before the `DOM` constant otherwise the `createOutputScreen()` function can't access any of them, and it does access one of them: `NUMBER_BUTTON_TEXT_NODES.zero.display`.

So the initialisation of this function should be done better, currently in `constants.js`:

``` js
import createOutputScreen from './createOutputScreen';

export const DOM = {
root: document.getElementById('calculator'),
outputScreen: createOutputScreen()
};
```