Included DS_Store files in .gitignore.

[jerieljan]

DS_Store files are commonly ignored since they're generated by macOS, and frequently encountered by Mac developers.

Merging this should prevent them from being pushed by a Mac user contributor in the future and cluttering up their git status.

example:

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	.DS_Store
	.github/.DS_Store
	extensions-builtin/.DS_Store
	modules/.DS_Store
	test/.DS_Store

[aliencaocao]

#4534 (comment)

[jerieljan]

Thanks for pointing that out. I appreciate it.
I personally disagree since I usually add them anyway (like here) to avoid stray edits, but that's fine.