Provide helpers for attachment and more

[daklik]

In Apostrophe Nunjucks, we have helpers in order to manage attachments like apos.attachment.all apos.attachment.first, apos.attachment.url and so on.
We would need those helpers to be available in Astro side in order to manage those attachments.
We are currently using our lib for that, it would be better to be exposed directly by this integration so as every projects would benefit from that.