Add some common objc nits to the style guide

[tadeuzagallo]

@nicklockwood@a2@vjeux Want to add something else? (or remove something)

[brentvatne]

@tadeuzagallo - to JS, we could add something like:

Leave a trailing comma after the list item in object literals
Use object destructuring assignment where possible

For objc, do you have a preference on using [x setVar:y] vs x.var = y? Might be worth picking one to avoid any future bike shedding.

[nicklockwood]

It's difficult to know what level of detail to go into with something like this. We do have very specific style guidelines for Objective-C code, but they'd probably run to a dozen pages if we mentioned everything!

The [x setVar:y] vs x.var = y is a good one (we prefer the latter, but only for @properties, not methods).

Besides that, may as well merge this as is and we can expand on it later as and when issues arise.