Support Twig chain loaders if available

[emodric]

One of our Symfony bundles that we're testing on Bolt uses a custom Twig loader.

This results in $this->getTwig()->getLoader() code used in Bolt in several places to return an instance of Twig\Loader\ChainLoader instead of Twig\Loader\FilesystemLoader making the code in question fail in Bolt, which results in Bolt not being able to find any of its templates.

This fix adds support for ChainLoader, which basically iterates over any Twig loader in the chain loader, and if it is an instance of FilesystemLoader, does what Bolt did previously (adds or prepends a new path, based on context).

Thanks to @ilukac for discovering the cause of the issue.

[bobdenotter]

I haven't bumped into this yet, but I can see how this would be problematic. Thanks for the fix, @emodric and @ilukac 👍

[emodric]

Thanks @bobdenotter !