BridgeJS: Export with a different JS name

[wfltaylor]

Allow using a different name in JavaScript for properties/methods. This is useful if a different name is more appropriate in JavaScript, or to export multiple overloaded Swift functions with distinct JavaScript names.

import JavaScriptKit

@JS("greetName") public func greet(_ name: String) -> String {
    return "Hello, \(name)!"
}

@JS("greetPerson") public func greet(_ person: Person) -> String {
    return "Hello, \(person.name)!"
}

exports.greetName("World");
exports.greetPerson({ name: "World" });

I decided to avoid having an argument label, to match @c and @objc more closely.