Calling getCollection from a preview template should expose the slugs

Currently, when calling `getCollection()` inside a preview template, only the data of the items is returned. In order to be useful, the returned Map should also include the path & slug.

Since the function already returns a Map (with just a single `data` key), adding path and slug is trivial and would be a non-breaking change.

I'll submit a PR for this.

Start by locating getCollection in the preview-template implementation and inspect where its returned Map is assembled. Confirm the current data-only shape, then verify that each returned item exposes its path and slug without breaking existing consumers.

Written by the indexing model from the issue text.