feat: Add twill:make:singleton command

[pboivin]

Description

This PR adds a new Artisan command to generate singleton modules.

Singletons are a type of module that are designed to only have a single record in the database. Unlike regular modules, they do not have an index page in the CMS. Typically, the menu entry for a singleton will lead directly to the form of its first and only record.

Singletons a great for one-off pages that require unique fields and/or custom logic (e.g. Home, About, Contact, etc.). An example from demo.twill.io :

Like twill:make:capsule, this new command is an extension of twill:make:module. Outside of the base command, some other modifications were made to support this feature:

• added a SingletonModuleController base class
• added a Route::singleton() macro
• added support for a singleton parameter for twill-navigation.php items
• added Seeder class stub for singleton modules
• added a few helper methods to handle stub file processing (this will be further improved in a future PR)

Some integration tests were also added along the way.

The command in action:

Related Issues

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[ifox]

This is amazing @pboivin! Very nice holistic solution for Twill singletons.

Minor nitpick: can the contactpage singleton in tests be renamed to contactPage?