Add NSExpression syntax for conditionals with multiple branches

[1ec5]

The NSExpression-based property value implementation in #10726 should support a more compact syntax for conditionals with multiple branches. Currently, an NSExpression containing a nested TERNARY gets converted into nested case expressions, and a case expression with multiple branches gets converted into a nested TERNARY structure. At least a nested TERNARY should be converted into a flat case.

[1ec5]

@friedbunny pointed out that +[NSExpression expressionForConditional:trueExpression:falseExpression:] is unavailable in iOS 8.x, so the proposed syntax would be even more useful for developers that need to deploy back to that version.

[friedbunny]

Indeed, besides recommending +[NSExpression expressionForConditional:trueExpression:falseExpression:] in the docs a couple times, we use it here:

mapbox-gl-native/platform/darwin/src/NSExpression+MGLAdditions.mm

Line 477 in fb5b8d3

return [NSExpression expressionForConditional:conditional trueExpression:trueExpression falseExpression:falseExpression];

and here:

mapbox-gl-native/platform/darwin/test/MGLExpressionTests.mm

Line 580 in 44ca0a5

NSExpression *expression = [NSExpression expressionForConditional:conditional trueExpression:trueExpression falseExpression:falseExpression];

[1ec5]

Fixed in #11450 on the release-boba branch for iOS map SDK v4.0.0 and macOS map SDK v0.7.0.