[13.x] Allow the exception renderer to have additional source paths

[SanderSander]

This PR adds a new method to the \Illuminate\Foundation\Exceptions\Renderer\Renderer class that allows developers to specify additional source paths so that files in those paths are not considered "vendor code".

This will make the source code rendered on the exception page, and therefore super useful for package development.

My motivation for this is that I link my packages into the vendor directory when developing them.

Example: (I linked ./vendor/laravel/framework into ./../framework), and tested on Windows/Linux.

/project
   /vendor/laravel/framework  (symlink to /framework)
/framework

Renderer::sourcePaths([
    base_path('../vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware'),
]);

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!