Update autoloader to push with namespace

- This prevents a strange bug wherein Zeitwerk would load the target
  gems files, but raise a Zeitwerk::Name error as it expected the file
  to define the constant, but it didn't
- This was fixed by specifying which namespace the pushed lib directory
  is part of during autoloading

lib/kangaru/application.rb

@@ -29,7 +29,7 @@ def run!(argv)
     def autoloader
       @autoloader ||= Zeitwerk::Loader.new.tap do |loader|
         loader.inflector = Zeitwerk::GemInflector.new(root_file)
-        loader.push_dir(app_dir)
+        loader.push_dir(app_dir, namespace:)
       end
     end
   end

sig/zeitwerk/loader.rbs

@@ -6,7 +6,7 @@ module Zeitwerk
 
     def collapse: (Array[String]) -> void
 
-    def push_dir: (String dir) -> void
+    def push_dir: (String dir, ?namespace: Module) -> void
 
     def setup: -> void
   end

spec/kangaru/application_spec.rb

@@ -47,7 +47,11 @@
 
     it "configures the loader to load the app dir" do
       setup
-      expect(loader).to have_received(:push_dir).with(application.app_dir).once
+
+      expect(loader)
+        .to have_received(:push_dir)
+        .with(application.app_dir, namespace:)
+        .once
     end
 
     it "enables the instantiated loader" do