This is a step towards removing object private slots.
Classes with JSCLASS_PRIVATE_IS_NSISUPPORTS now use JSCLASS_SLOT0_IS_NSISUPPORTS
instead. For most classes this means we need to add an extra reserved slot and remove
the private slot.
Global objects (SimpleGlobalObject and the XPConnect BackstagePass and Sandbox globals)
however can use the JSCLASS_GLOBAL_APPLICATION_SLOTS already there. These slots were
only used for WebIDL DOM globals until now.
Differential Revision: https://phabricator.services.mozilla.com/D119502
59f8590115