Pagination: pass metadata to before callback

[vseventer]

This PR adds a second argument for passing page data in the paginations' before callback.

This is useful (especially when leveraging the Data Cascade, where a piece of metadata might be elsewhere in the cascade) when you want to process (filter or modify) based on a value in the frontmatter:

---js
{
  keyword: "item3",
  pagination: {
    data: "items",
    size: 1,
    before: function(data, metadata) {
      return data.filter(el => el === metadata.keyword);
    }
  },
  items: ["item1", "item2", "item3", "item4", "item5", "item6"]
}
---
{{ pagination.items[0] }} <- item3

[vseventer]

Hi @zachleat - what is the process for getting this looked at? If there's anything I can do, let me know!

[MadeByMike]

Makes sense to me

[zachleat]

Some commentary at #1522

[zachleat]

Shipping with 1.0.1, earliest canary is 1.0.1-canary.4