"Skip to content", invisible until focused.

## Usage

```vue
<DuxtSkipLink />
```

## API

No props.

## Notes

Without it every page change means tabbing through the header, the section row
and the whole sidebar before the first word of the text. `sr-only` collapses it
to a single pixel, and the focus state puts it back on screen above everything
else.

The click is handled rather than left to the hash, because the browser only moves
focus for a hash link in some browsers — the handler moves it in all of them.