* Added "Invokable Cases" PHPStan extension
This includes a PHPStan extension to add support for `InvokableCases`
so that static analysis tools can understand the callable methods and
their return types.
The extension has been added in a way that will allow multiple extensions
in the future if required, with a single include file that will import
all extensions, or the option to include only specific extensions.
* Added "Invokable Cases" PHPStan extension testing
