feat: Add target attribute to links in listing templates if provided

[mcanouil]

Enhance the listing templates by conditionally adding a target attribute to links. This allows for better control over link behaviour, enabling users to open links in new tabs if specified. This change improves user experience by providing more flexibility in navigation.

I'm not adding it yet to changelog as it might not be for 1.7, I'll also add a test if this Poc is a good idea.

---
title: "12276"
listing:
  contents:
    - path: posts/post-with-code/index.qmd
      target: _blank
    - path: posts/welcome/index.qmd
  sort: "date desc"
  type: default
  categories: true
  sort-ui: false
  filter-ui: false
page-layout: full
title-block-banner: true
---

Screen.Recording.2025-03-14.at.21.43.08.mov

Originally discussed in:

• How to open listing items in a new tab when using reveal.js slide shows? #12276