Replace step_dir expression context with func_dir

Changes

This change updates how the function directory is obtained in the step-runner system. Previously, users had to reference the directory using ${{ step_dir }}, but now they must use the ${{ func_dir }} syntax instead. The old step_dir syntax still works as a fallback for backward compatibility.

Reference

Resolves issue Rename ${{step_dir}} to ${{func_dir}} for GitLa... (#339 - closed)