Support for GitLab CI Functions (.gitlab-ci.yml)

### Feature Request: Support for GitLab CI Functions GitLab recently introduced [CI Functions](https://docs.gitlab.com/ci/functions/), which allow users to modularize and reuse pipeline logic in their `.gitlab-ci.yml` files. Supporting CI Functions in `gitlab-ci-local` would greatly increase compatibility with upstream GitLab CI and unlock new workflows for users. **What is requested:** - Add support for parsing, evaluating, and executing CI Functions within `.gitlab-ci.yml` files, matching GitLab’s upstream implementation as closely as possible. - Ensure CLI usage and the local execution experience mirrors real GitLab CI/CD behavior with respect to functions. - Add targeted tests for pipelines using CI Functions, following the established `tests/test-cases/.../` structure (do not run full `bun test`). - If upstream coverage is incomplete or ambiguous, document the limitations in the README or as user-facing warnings. - Consider configurable flags to enable/disable experimental or partial CI Functions support, if full compatibility requires significant refactoring. **Reference:** - GitLab CI Functions documentation: https://docs.gitlab.com/ci/functions/ **Motivation:** - Closer feature parity with GitLab CI/CD - Reduced workflow friction for users adopting CI Functions - Fewer surprises when pipelines are pushed to GitLab --- Label: `feature` (please tag appropriately) Thanks for your work on this project!