Casting an object to an array does not have the intended effect of
wrapping the object in an array. Instead we need to explicitly check
if the returned value is an array or not.
The gathering and execution of extenders can actually be done here
in the `Extension` class. This way, the `ExtensionManager` only
deals with the question of which extensions are enabled, the
`Extension` class actually extends the core application, and the
service provider simply calls a method, without having to know
about internals.
Loading the activated extensions now means retrieving an array of
extenders (classes that implement a certain type of extension of a core
feature in Flarum).
For now, the only existing extender is the Compat extender which is used
to handle old-style bootstrappers that simply return a closure that
receives all of its dependencies via auto injection.
In the future, extensions will be able to return an array of extender
instances from their bootstrapper instead. These extender classes will
be implemented in the next step.
