feat: support Twig 3 alongside Twig 2

[austinderrick]

Summary

Widens the twig/twig dev requirement from ^2.11 to ^2.11 || ^3.0 so Assetic's Twig integration is exercised against Twig 3, and resolves the incompatibilities and deprecations Twig 3 surfaces. All changes are backward compatible with Twig 2.x, so existing consumers on Twig 2 are unaffected.

This also removes the first-party reason CI saw deprecation noise on PHP 8.4/8.5: the Implicitly marking parameter … nullable notices came from the old Twig 2.x line, which is clean on Twig 3.

Changes

• composer.json — twig/twig: "^2.11 || ^3.0". Composer resolves the highest compatible version, so CI runs against Twig 3 on PHP 8.x while Twig 2 stays supported.
• src/Assetic/Extension/Twig/AsseticTokenParser.php — Twig 3 lexes the opening [ of a vars=[...] attribute list as an operator token rather than punctuation. The parser now accepts either, so {% javascripts … vars=["foo","bar"] %} parses on both majors. (, and ] are punctuation in both and are unchanged.)
• src/Assetic/Extension/Twig/AsseticNode.php — two Twig 3 deprecations in first-party code:
  • Twig 3.9 deprecated nodes that compile to echo instead of yield. AsseticNode only assigns to the context and subcompiles its body (it never emits echo), so it is yield-safe — marked with #[\Twig\Attribute\YieldReady]. The attribute is inert on Twig 2.x (never resolved) and on PHP 7.x (parses as a comment).
  • Twig 3.12 deprecated passing the node tag to the Node constructor (the Parser sets it automatically now). The tag is no longer forwarded on Twig ≥ 3.12, while Twig 2.x / < 3.12 still receive it.
• tests/Assetic/Test/Extension/Twig/AsseticExtensionTest.php — the internal Environment::loadTemplate() signature changed in Twig 3; the test helper now uses the public load() method, which returns a renderable TemplateWrapper on both Twig 2.11+ and 3.x.

Verification

Ran the full suite with the symfony/phpunit-bridge deprecation helper enabled (the default) — no failures, no errors, and no remaining first-party deprecations — against both Twig versions:

# PHP 8.3 + twig/twig v3.27.1  →  exit 0
# PHP 8.3 + twig/twig v2.16.1  →  exit 0
OK, but incomplete, skipped, or risky tests!
Tests: 408, Assertions: 547, Skipped: 61.

(The earlier push failed CI because the bridge correctly flagged the two first-party Twig 3 deprecations above; both are now fixed at the source rather than suppressed.) phpcs (PSR-12) is clean on the changed files.

Notes

Independent of #48 (which adds 8.4/8.5 to the CI matrix and quiets the unfixable vendor deprecations); the two PRs touch disjoint files and can merge in any order. Dropping Twig 2 entirely (^3.0 only) was deliberately avoided to preserve backward compatibility, in keeping with Assetic's wide version support.