Extend NSExpression with convenience methods

[1ec5]

As a more structured alternative to #11015, we should extend NSExpression with convenience methods and initializers that represent common operations for which #10726 uses the custom function syntax:

NSExpression(forConstantValue: "Old")
     .mgl_appending(NSExpression(forConstantValue: " "))
     .mgl_appending(NSExpression(forConstantValue: "MacDonald"))

This is the most conventional approach, but it’s also very verbose – even moreso in Objective-C – and difficult to use in conjunction with format strings.

#5970 (comment) described essentially the same approach for the MGLStyleValue API that #10726 tore out.

/cc @akitchen @fabian-guerra @jmkiley

[1ec5]

Fixed in #11278 on the release-boba branch for iOS map SDK v4.0.0 and macOS map SDK v0.7.0.