Pre render for quarto preview

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/4822

^{Originally posted by itepifanio March 14, 2023}
Quarto provides pre and post rendering features but it doesn't extend when live previewing the documents.

I have a script that reads yaml content and outputs .qmd files used in a Quarto website. I was trying to add a custom watcher for the yaml files, this way, when a yaml file changes the .qmd files the Quarto live previewing would refresh.

My solution was running Quarto as a subprocess and creating the custom watcher. A pre rendering that works with live previewing would make my life easier because I would be able to run the yaml watcher script without working with subprocessing.

Start by tracing the Quarto preview path and how pre-render scripts and file watchers are currently handled. Verify the behavior described in the issue: pre-rendering should run during live preview, and changes to watched YAML files should refresh the generated .qmd files and preview without requiring a subprocess.