Support `Iterable` on `NSArray`

[stuartmorgan-g]

This is related to #1444, but broader in scope.

I wanted to convert some code that was using method channels and did a map(_helperFunctionToConvertElements), and the corresponding code with FFI was a very old-school C-ish style loop-indexes-from-0-to-length-minus-one to manually both convert each element of the array, but also convert the NSArray to a List. If NSArray supported Iterable, then it would have been simple and idiomatic to do the conversion to FFI.

[stuartmorgan-g]

(After filing this I realized that since I explicitly wanted a List in my use case I could make a helper utility that would convert the NSArray to a generic List and then do the rest of the conversions with Dart-idiomatic map operations, but having that built in to objective_c, especially for cases where someone just needs an Iterable and not a List, would be still be a good quality-of-life improvement.)

[liamappelbe]

@stuartmorgan-g What about nullability? Should NSArray implement Iterable<ObjCObjectBase> or Iterable<ObjCObjectBase?>

[stuartmorgan-g]

The standard Objective-C collections cannot contain nils.