build: vendor metalua and stringutils as tracked files - #49

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/upload-artifact@v4, extractions/setup-crate@4993624604c307fbca528d28a3c8b60fa5ecc859. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/