Add .DS_Store to Swift, ObjC & Xcode gitignores

[ChrisMash]

Reasons for making this change

I've spent years manually adding .DS_Store to every .gitignore I've created from Swift.gitignore so those (seemingly) meaningless files don't get included, maybe I'm missing a reason that they've been left in, though they're excluded from the Global/macOS.gitignore, so assume they should also be excluded in these templates?

I assume it's ok to "ignore" the contribution guideline regarding only one template change per PR, seeing as the templates being changed say to also change the other ones? If not I can raise them separately, but it seems relevant to keep them all in one PR for a single change to each like this.

Links to documentation supporting these rule changes

Hope this suffices, let me know if not!

Merge and Approval Steps

• Confirm that you've read the contribution guidelines and ensured your PR aligns
• Ensure CI is passing
• Get a review and Approval from one of the maintainers

[ChrisMash]

Not sure how CI gets run, it doesn't seem to be happening automatically and I can't find anywhere that says how to trigger it or see any actions that obviously relate to it 👀

[wirecat]

We don't currently accept OS specific rules in our templates. We encourage people to instead use global gitignore configs and have a collection of them that can be used. Please see our Global/README for more information and Global/macOS.gitignore for an example.