Streamline custom function syntax for NSExpression

[1ec5]

As of the initial NSExpression implementation in #10726, we rely on the clunky custom function syntax to represent many of the style specification’s expression operators. For example, string concatenation is a common operation by requires all sorts of undiscoverable and error-prone punctuation:

NSExpression(format: "FUNCTION('Old', 'stringByAppendingString:', FUNCTION(' ', 'stringByAppendingString:', 'MacDonald'))")

As previously proposed in #8074 (comment), we should extend the private _NSPredicateUtilities class using the Objective-C runtime:

NSExpression(format: "mgl_join({'Old', ' ', 'MacDonald'})")

This would result in the most natural usage, but it’s also a bit Clever™. We’ll need to tiptoe around a bit to avoid directly using any private APIs, and we’ll need to include safeguards in case the underlying NSExpression implementation changes at any point.

/cc @akitchen @fabian-guerra @jmkiley

[fabian-guerra]

Fixed in #11472