Add inertia_rendering? method and set instance variable in renderer

[Andy9822]

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.

P.S. ideally, this could be subtly mentioned somewhere in the docs but I'm new here so I'll leave that up to you (although Im open to add it if you have a suggestion on where and how to word it if you want)

Closes #208

[Andy9822]

I'm not completely sure with the testing approach I've implemented for this helper. The current test directly extends the controller with InertiaRails::Helper, but this doesn't accurately reflect how the helper is actually included in a Rails application.

In the real application, the helper is included indirectly through InertiaRails::Controller, which is then injected into ActionController::Base via Rails' on_load hook during initialization.

[skryukov]

lgtm, @bknoles could you help merging this?

[bknoles]

Merged! I'd like to release after we get the deep merge stuff merged in!