Make screenshots script needs to be functionalized and added into ottrpal

[cansavvy]

What needs to be done:

make-screenshots.R needs to be functionalized and added to the ottrpal package (along with a unit test).

Currently its called like this:

Rscript --vanilla make_screenshots.R \
            --git_pat ${{ secrets.GH_PAT }} \
            --repo $GITHUB_REPOSITORY \
            --output_dir resources/chapt_screen_images

So each of these options should become arguments in the new function.

After an initial PR that adds this as a function (along with a test in both test-rmd-leanpub-prep.R and test-quarto-leanpub-prep.R

Then later:

• This change in the ottrpal package would need to be updated in the base_ottr image
• Then this job in the render_all.yml needs to be updated to use the base_ottr image and the new ottrpal function.