Configurable target namespace for translated pages
Introduce wgContentTranslationTargetNamespace variable
for configuring where to publish translated articles.
By default, publish in the Main namespace.
Added utility methods in JS and PHP sitemapper modules to support this.
Bug: T76618
Bug: T78229
Change-Id: I3801e82edff4c2f39531a76a3e98fc9dada7e0c0