Refactor gutter architecture with separate container views and improved marker layout

- Introduced STGutterContainerView, STGutterMarkerContainerView, and
  STGutterSeparatorView for clearer view hierarchy and separation of concerns.
- Moved separator rendering from STGutterView to STGutterSeparatorView.
- Added textSize to STGutterLineNumberCell to assist marker positioning.
- Improved layout logic to position markers based on associated line number
  cells.
- Updated STTextView to refresh marker layout in synchronization with line
  numbers.
- Adjusted default insets and minimum width for better appearance.