Fixes #3535: Syntax changes

- Removed `form_begin`, `form_end`
- Added `use()` and `ViewRenderer::uses` that are importing classes and namespaces
- Added widget dynamic functions `*_begin`, `*_end`, `*_widget`, `widget_end`
- Added more tests
This is allowed name, but I think it should be some kind of identifier because it's used later to access the extension in template: `$this->env->getExtension('Twig Extension for Yii 2')->registerAsset($context, "app");`