Add code auto-linking to release posts

[Bisaloo]

From #166 (comment). cc @joshwlambert.

This is possible via downlit package, which is used by quarto when rendering a rmarkdown document (source code).

A couple of things need to happen for this to work here:

• code-link: true needs to be added to the release post template YAML metadata header
• the release post template needs to load the package of interest
• the xml2 & downlit packages need to be added to renv.lock
• the correct version of the package of interest needs to be added to renv.lock by the automate-release-post workflow
• the workflow also needs to now pre-render and commit the related _freeze file to ensure netlify preview works.

Important note: code auto-linking will not appear in the netlify preview but it should still work in the final version when rendered from main!