Adding new function to Nunjucks
In [documentation ](https://eggjs.org/en/core/i18n.html#use-in-view) we have

```html
<li>{{ __('Email') }}: {{ user.email }}</li>
<li>
  {{ __('Welcome back, %s!', user.name) }}
</li>
<li>
  {{ __('Hello {0}! My name is {1}.', ['foo', 'bar']) }}
</li>
```

I want to know how you add `__` to the Nunjucks, I need to add a `t` function for my localizations
Can you please show me the source code?