fix: prevent include/extends escaping the configured template directory

[BrianWillows]

Summary

Twig.path.relativePath() resolves an {% include %} / {% extends %} /
{% embed %} target without checking that the result stays inside the
configured template directory. The resolver walks .. segments and, once the
running path is exhausted, simply pushes them:

} else if (val === '..' && newPath.length > 0 && newPath[newPath.length - 1] !== '..') {
    newPath.pop();
} else {
    newPath.push(val);   // '..' kept -> climbs out of the template root
}

So enough ../ segments escape the views/templates directory and read an
arbitrary file, whose content is inlined into the rendered output
(CWE-22 path traversal → CWE-98 local file inclusion).

Reproduced on 3.0.0 — a template inside views/ containing
{% include "../SECRET.txt" %} rendered the contents of a file placed
outside views/.

This is exploitable wherever the include target is influenced by untrusted
input: dynamic template/partial selection, a template name taken from a
request, or a user-supplied template. Static includes in trusted templates are
unaffected.

For reference, the PHP implementation's FilesystemLoader explicitly refuses
these paths ("Looking outside the configured directories is forbidden"), so
this is a divergence from the upstream security model rather than intended
behaviour.

Fix

When a template directory has been configured (template.base), reject targets
that resolve outside it.

Relative includes within the configured root keep working — including the
repo's own test/templates/include/relative.twig fixture
({% include "../simple.twig" %} from a subfolder), which the first version of
this patch broke and which drove the final approach. Only paths that climb out
of the root are refused.

Verification

• Full test suite passes: 533 passing, 0 failing (mocha -r should).
• With base set to a views/ directory:
  • {% include "../SECRET.txt" %} → refused, nothing leaked.
  • {% include "../partial.twig" %} from views/sub/ → still renders.
  • same-directory include → still renders.

Note / question for maintainers

When the include is refused, the thrown Twig.Error currently surfaces through
the async path as a secondary TypeError: Cannot read properties of undefined (reading 'valueOf') rather than cleanly propagating. That's the existing error
plumbing in twig.async.js / twig.core.js rather than something this patch
introduces, but I'm happy to follow up so the rejection reports cleanly — let me
know how you'd prefer it handled.

Found and fixed with AI assistance (Claude). Happy to add a regression test.