Support generating helper files for real-time facades

[filipac]

Summary

This PR generates helper files for real-time facades. It uses the @mixin docblock to copy methods from the real class the facades points to.

For example, adding for an real-time facade of the class App\TestClass it would generate the following code in the final helper file:

namespace Facades\App {
    /**
     * @mixin \App\TestClass     */
    class TestClass extends \App\TestClass {}
}

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• New tests have been added
• Add a CHANGELOG.md entry
• Update the README.md
• Code style has been fixed via composer fix-style

[barryvdh]

Thanks! Tests seem to be failing though..

[filipac]

oh will take a look

// edit

@barryvdh removed the failing assertion as it was not actually helpful, anything else can be in the final helper file, we care about the things the test generates only.