hugolib: Add zero-based Ordinal to shortcode

hugolib: Add zero-based Ordinal to shortcode

The count starts at 0 relative to the shortcode's parent: Either the page or the surrounding shortcode.

Access it in a shortcode like this:

```bash
Ordinal is {{ .Ordinal }}
```

Note that this is a shared ordinal for all shortcodes in the relevant context, so, as an example, you have this in a content page:

```markdown
This is a shortcode:

{{< hello >}}

This is another shortcode:

{{< hugo >}}

The `.Ordinal` you get in the two shortcodes above is 0 and 1.
```

See #3359