Add snapshot tests for STTextView and integrate SnapshotTesting on macOS

- Added the swift-snapshot-testing dependency to capture view snapshots
- Created STTextViewSnapshotTests covering text rendering, font styling,
  selection, editable state, color, and view size variations
- Generated and committed image snapshots under __Snapshots__ directory
- Adjusted Package.swift to include SnapshotTesting in macOS test targets