Build frontend boilerplate code for Commit list refactor

Summary

As part of the Commit List refactor, we want to lay the groundwork for the new Vue-based frontend. This issue covers the initial boilerplate setup, including the feature flag, JS entry point, and placeholder Vue components. This will allow us to incrementally implement and test features behind a feature flag without impacting the current HAML-based implementation.

Implementation Tasks

• Add a new feature flag (e.g. :repository_commits_refactor)
• Create a new JS entry point element for the new Vue app in show.html.haml: (e.g. #js-commit-list)
• Set up a Vue root component (commit_list_app.vue) with basic structure
• Render Vue app behind the feature flag
• Create placeholder components for:
  • <CommitListApp />
  • <CommitItem />
  • <CommitDetails />
• Pass down dummy props to structure the basic layout
• Add basic tests