Add configurable file theme icons

[trin4ik]

Adds a configurable icon style for the file tree.

By default the tree keeps the existing behavior and shows the extension's git status icons for files. A new gitTreeCompare.iconStyle setting can switch the tree to VS Code file theme icons instead:

• status -> existing git status icons
• fileTheme -> file/folder icons from the active VS Code icon theme
• in fileTheme mode, file labels are colored by git status
• untracked files are gray, modified files are blue, added files are green and deleted files red colors

This keeps the current UI unchanged for existing users while allowing the compare tree to visually match the Explorer file tree when desired.

2026-06-18.20.08.16.mp4