Support Swift Package Manager

[harumidiv]

Overview

Adds Swift Package Manager support, so the library can be consumed via SwiftPM
alongside the existing CocoaPods and Carthage integrations.

Changes

• Package.swift — declares a single GradientCircularProgress library target
  that builds the existing source directory through path:. No files were moved
  or renamed, so CocoaPods (source_files = "source/**/*"), Carthage, and the sample
  Xcode project all keep working exactly as before. The Objective-C umbrella header
  is excluded, as a SwiftPM Swift target does not use one.

• import UIKit added to Progress/Property.swift and
  Progress/Elements/Background.swift — these were the only two files without an
  explicit import. In the framework target they resolved CGFloat, UIColor, and
  UIVisualEffectView through the umbrella header (GradientCircularProgress.h);
  SwiftPM has no umbrella header, so the build failed with
  cannot find type 'CGFloat' in scope. The same change is mirrored into
  GCProgressSample/GradientCircularProgress/ to keep the two copies of the sources
  in sync.

• README.md — SwiftPM installation instructions.

• .gitignore — SwiftPM build artifacts (.build/, .swiftpm/, Package.resolved).

Verification

• xcodebuild -scheme GradientCircularProgress -destination 'generic/platform=iOS Simulator'
  → BUILD SUCCEEDED, with no warnings (Xcode 26.3).
• Swapped a real app's dependency from the CocoaPods integration over to this branch
  via SwiftPM. Both the app and its test target build, and all 1,406 library symbols
  link into the app binary with zero undefined symbols.

Note on releasing

SwiftPM resolves versions by tag, so a new tag (e.g. 3.14.0) would need to be pushed
after merging before anyone can actually depend on this. The README documents
from: "3.14.0" on that assumption — happy to change it to whatever version number
you'd prefer.