Add inertia_rendering? method and set instance variable in renderer

We are introducing a new method `inertia_rendering?` in the `InertiaRails` helper to check if the controller is currently rendering an Inertia response.
This allow a view to conditionally render something based on this option i.e. the layout may include the `inertia` pack based on this.

Author: Andy9822

lib/inertia_rails/helper.rb

@@ -11,4 +11,8 @@ def inertia_headers
     )
     inertia_ssr_head
   end
+
+  def inertia_rendering?
+    controller.instance_variable_get("@_inertia_rendering")
+  end
 end

lib/inertia_rails/renderer.rb

@@ -37,6 +37,7 @@ def initialize(component, controller, request, response, render_method, props: n
       @deep_merge = deep_merge.nil? ? configuration.deep_merge_shared_data : deep_merge
       @encrypt_history = encrypt_history.nil? ? configuration.encrypt_history : encrypt_history
       @clear_history = clear_history || controller.session[:inertia_clear_history] || false
+      @controller.instance_variable_set('@_inertia_rendering', true)
     end
 
     def render