Add tag to partially enable strict mode

### Use cases
 - Converting a non-strict twig codebase to a strict codebase by partially enabling strict mode on a single template base
 - Enabling strict mode only for certain important templates that have a higher breakage potential

This PR introduces a new tag `{% strict %}` and `{% endstrict %}` to enable strict mode on a block base. It behaves recursively in a way that includes, embeds, etc. in this block are evaluated with strict mode enabled as well.