feat: Make the parallel rendering robust

Description

Right now the parallel rendering isn't reliable and returns error because it doesn't find the mediabag folder.

future::plan(future::multisession, workers = 8)

for (i in 1:100){
  write(file = sprintf("test%s.qmd", i),
        sprintf("---
title: 'test%s'
---

## Quarto is cool", i)
  )
}

list.files(pattern = "\\.qmd$", full.names = TRUE, recursive = TRUE) |>
  squash::compile_qmd_course(
    output_dir = tempdir(),
    output_html = "complete_course.html"
  )

ℹ {future} is using future::plan(future::multisession, workers = 8), to modify this use `future::plan()`
✖ Failed to render ./test18.qmd, cleaning and existing
✖ Failed to render ./test40.qmd, cleaning and existing
✖ Failed to render ./test45.qmd, cleaning and existing
✖ Failed to render ./test47.qmd, cleaning and existing
✖ Failed to render ./test54.qmd, cleaning and existing
✖ Failed to render ./test74.qmd, cleaning and existing
✖ Failed to render ./test75.qmd, cleaning and existing
✖ Failed to render ./test76.qmd, cleaning and existing
✖ Failed to render ./test78.qmd, cleaning and existing
✖ Failed to render ./test87.qmd, cleaning and existing
✖ Failed to render ./test95.qmd, cleaning and existing
Error in squash::compile_qmd_course(list.files(pattern = "\\.qmd$", full.names = TRUE,  : 
  Failed to render all qmd files.

Diagnostic is that the folder is created by one process, listed by the second, then deleted by the first, and unreachable by the second

Start by reproducing the provided future::multisession example and inspect the compile_qmd_course entry point, focusing on how the mediabag folder is created, listed, and deleted across workers. Done means the example reliably renders all QMD files in parallel without workers losing access to the mediabag folder.