Documentation: twig.function.* option

[fvsch]

Document the new way to expose a function in the templating environment:

c::set('twig.function.myFunction', 'myFunctionsFullName');
c::set('twig.function.myFunction', function(){ /* return something */ });

And similarly for filters.

The old way, c::set('twig.env.functions', ['myFunctionsFullName']);, still works.

This feature enables plugins to register Twig functions and namespaces:

c::set([
  'twig.function.doSomething' => 'MyPlugin::doSomething',
  'twig.namespace.myplugin'   => __DIR__ . '/templates'
]);