Simplify retrieval of collections.all in getLocaleCollectionItem

[noelforte]

This change motivated by: noelforte/eleventy-plugin-vento#221

I couldn't figure out why the i18n plugin's presence was breaking the getNextCollectionItem and getPreviousCollectionItem filters since the Vento template engine plugin doesn't do anything fancy with the filters.

After a lot of trial and error, I discovered that this search for collections.all at the very end of the GetLocaleCollectionItem module checks a lot of things specific to Eleventy's own internal engines but of course custom engines don't add a ctx or context object, or even this.collections, since this.page and this.eleventy are the only context keys Eleventy supports, and collections is already accessible in page data anyway.

Rather than add this.collections to the Vento custom engine, could we simplify the all declaration to search the data key instead since the data key is already available on the render context anyway?

If not, I can look into adding this.collections to the custom engine for my plugin, but if this can be handled upstream that puts less pressure on custom engines to support adding it.

Thanks Zach!

[noelforte]

Sorry to bug you Zach, any inclination on when this might get reviewed? Is there something that I could do to help push things along?

[noelforte]

@zachleat Just confirming, is this PR on hold for a specific reason? It's not been tagged as on-hold, and also I haven't heard anything about getting this reviewed apart from one reference in #2844. I know you're busy but I'd like to know whether I should be implementing a workaround on my own or whether the 11ty codebase can adapt these changes.