Remove tracked .DS_Store / editor detritus from the repo

[d-morrison]

The default branch (main) carries OS/editor detritus as tracked files:

• .DS_Store
• figures/.DS_Store

These serve no purpose in the repo and reappear in every clone and diff.

Fix, in this repo:

git rm --cached .DS_Store figures/.DS_Store
printf '.DS_Store\n' >> .gitignore

Fix, per-machine, so it stops recurring:

git config --global core.excludesFile ~/.gitignore
printf '.DS_Store\n' >> ~/.gitignore

R users can do the same in one call, which also covers .Rproj.user, .Rhistory, .Rdata, .httr-oauth, and .quarto -- see https://usethis.r-lib.org/reference/git_vaccinate.html:

usethis::git_vaccinate()

Found while scoping Morrison-Lab/gha#605, which proposes a reusable CI check for exactly this.