Introduce empty file tree with feature flag in the pipeline editor

What does this MR do and why?

For #350721 (closed)

This is the first step towards adding a file tree to the pipeline editor. This MR introduces the feature flag (pipeline_editor_file_tree) and the empty file tree.

The file tree will only show up when the feature flag is turned on and if there a non-empty config file already exists on the branch. It will not show up during the first time the user creates a config file through the pipeline editor, since we don't want to distract them from the onboarding process with advanced features.

Screenshots or screen recordings

State	Screenshot/Recording
With Feature Flag	file_tree
With Feature Flag, Empty State	empty_state
No Feature Flag
No Feature Flag, Empty State

How to set up and validate locally

1. Enable the feature flag.

   Feature.enable(:pipeline_editor_file_tree)

2. Visit the pipeline editor (CI/CD > Editor)
3. Verify that the file tree exists.

MR acceptance checklist

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