Support Swift Package

[bcylin]

Added Package.swift that specifies two separate libraries/targets due to different source file paths:

• Source/iOS+tvOS/**/*
• Source/macOS/**/*

Therefore, selecting the platform-specific library is required when adding the package to a project.

Introduce Package.swift so Hue can be added to Xcode projects using Swift Package Manager:

[codecov-io]

Codecov Report

Merging #70 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #70   +/-   ##
=======================================
  Coverage   93.13%   93.13%           
=======================================
  Files           4        4           
  Lines         539      539           
=======================================
  Hits          502      502           
  Misses         37       37

Impacted Files	Coverage Δ
Source/iOS+tvOS/UIImage+Hue.swift	97.88% <ø> (ø)	⬆️
Source/iOS+tvOS/UIColor+Hue.swift	77.02% <ø> (ø)	⬆️

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