Support custom Locale when using iOS 15 compatible String initialiser

[liamnichols]

Unlike LocalizedStringResource, the properties within the generated String.Localizable type remain private to the generated source and cannot be directly accessed.

This causes an issue if you need to be able to change the Locale value to something other than .current.

To address this, I've updated String.init(localizable:) to be String.init(localizable:locale:), with a nullable value.

If the custom locale is nil (the default value), we will use the original locale that is supplied by code generation.

If a value is set, it will take priority and allows you to override the language returned, for example:

String(localizable: .greeting) // Hello
String(localizable: .greeting, locale: Locale(identifier: "fr")) // Bonjour