Twig: add asset() function for assets
Service: add Session
Service: add Form
Twig: remove redundant debug option

The debug option syncs with App config in TwigServiceProvider
Service: add Validator
Translation: improve translation resource handling

Now Translator properly handle both `<locale>.yml` and `<domain>.<locale>.yml`.
Validator: add empty translation resources
Translation: rename default resource file to <domain>.<locale>.yml