`.DS_Store` should be ignored in `.gitignore`

[emilykl]

.DS_Store utility files generated by Mac OS should be ignored in .gitignore because we never want to commit them to the repo. Without ignoring, git status output looks like this:

$ git status
On branch master
Your branch is up to date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        packages/.DS_Store
        packages/python/.DS_Store
        packages/python/chart-studio/.DS_Store
        packages/python/chart-studio/chart_studio/.DS_Store
        packages/python/plotly-geo/.DS_Store
        packages/python/plotly-geo/_plotly_geo/.DS_Store
        packages/python/plotly/.DS_Store
        packages/python/plotly/_plotly_utils/.DS_Store
        packages/python/plotly/codegen/.DS_Store
        packages/python/plotly/jupyterlab_plotly/
        packages/python/plotly/plotly/.DS_Store