Clean up & Cocoapods removal & Metadata fix

Added

• Support for app icon appearance variants (Dark and Tinted). Icon entries with
  appearances in Contents.json are now matched separately between AppIcon
  and AppIconOriginal, so each variant receives its own overlay.
• Tests covering appearance-variant matching and duplicate icon metadata.
• README: table of contents, parameter reference tables, a full Tuist-based
  example, and an Examples gallery showing every --versionStyle
  (Documentation/Icons/*.png).

Changed

• Package now requires Swift tools 6.0 and macOS 14. The target is declared as
  an executableTarget, dependencies use explicit .product(...) references,
  and the language mode is pinned to Swift 5.
• --resources is now required. The fallback to $PODS_ROOT/VersionIcon/Bin
  was removed.
• Any unhandled error now exits with code 1 instead of the error's internal
  code, so failures reliably stop the build.
• Requirements updated to macOS 14+ on Apple silicon (arm64-only binary) and
  Xcode 16+.
• README rewritten around Swift Package Manager installation. Badges now point
  at GitHub tags and the repo license; images are hosted in the repo instead of
  an external image host.
• Prebuilt Bin/VersionIcon rebuilt with the changes above.

Removed

• CocoaPods support: VersionIcon.podspec, the Pods/ .gitignore entry, and
  the CocoaPods installation instructions.
• Old Documentation/AppIcon60x60@2x.png and @3x.png sample images.