Add cache_tracker tag

[aerni]

Adds a {{ cache_tracker }} tag for registering a tracked tag directly from a view, instead of rendering an empty dummy partial just to trigger the partial hook.

{{ cache_tracker tag="page_builder:projects_latest" }}

{{# or as the tag itself #}}
{{ cache_tracker:page_builder:projects_latest }}

This is useful, for example, for a projects block with two modes. A latest mode pulls the newest entries via a {{ collection }} query, so its output depends on the collection's order. A selection mode uses hand-picked entries. Tracking only the latest variant previously meant a dummy partial. Now it's inline:

{{ if block:query == 'latest' }}
    {{ cache_tracker tag="page_builder:projects_latest" }}
{{ /if }}

The latest variant gets its own tag so it can be invalidated on its own in a CollectionTreeSaved listener when the collection is reordered, while selection blocks stay cached.

[ryanmitchell]

Nice idea, I should have thought of this!