[feature/twig] Support using Twig filters on {VAR}, add masks for Twig tags

Now we can do {L_TITLE|upper}, {SITENAME|lower}, etc

We can also use all the Twig tags in our own syntax. E.g. <!-- BLOCK foo -->
= {% block foo %]. All tags are the same as the Twig tag names, but are
in uppercase.

PHPBB3-11598