Add SPM Support

[jdanek4]

• Added Swift.package file with necessary configuration
• Fixed unit tests to work using swift test command

Building:

swift build
[35/35] Build complete!

Testing:

➜  SKTiled git:(spm-support) ✗ swift test 2>&1 | xcbeautify
All tests
Test Suite SKTiledPackageTests.xctest started
ColorTests
    ✔ testColorHexAlphaEquality (0.002 seconds)
    ✔ testColorHexEquality (0.000 seconds)
    ✔ testColorMatchingHelpers (0.000 seconds)
    ✔ testColorParsingExtensions (0.001 seconds)
    ✔ testShortHexStringColorEquality (0.000 seconds)
CompressionTests
    ✔ testGZipCompressedString (0.000 seconds)
    ✔ testZlibCompressedString (0.000 seconds)
CoordinateTests
    ✔ testLayerCoordinateConversion (0.060 seconds)
    ✔ testLayerQueryAllCoordinates (0.023 seconds)
    ✔ testLayerQueryAtCoordinate (0.031 seconds)
    ✔ testTilemapCoordinateConversion (0.036 seconds)
    ✔ testTilemapQueryAtCoordinate (0.030 seconds)
ParserTests
    ✔ testMapExists (0.033 seconds)
    ✔ testMapHasCorrectFlagsSet (0.028 seconds)
    ✔ testMapIsUsingDelegates (0.038 seconds)
PerformanceTests
    ◷ testLargeMapLoadTime measured (0.000 seconds)
    ✔ testLargeMapLoadTime (0.302 seconds)
    ◷ testSmallMapLoadTime measured (0.000 seconds)
    ✔ testSmallMapLoadTime (0.256 seconds)
PropertiesTests
    ✔ testDoubleArrayProperties (0.074 seconds)
    ✔ testGetTilesWithProperty (0.034 seconds)
    ✔ testIntArrayProperties (0.043 seconds)
    ✔ testPropertiesAreIgnored (0.074 seconds)
    ✔ testStringArrayProperties (0.034 seconds)
QueryTests
    ✔ testQueryFunctions (0.036 seconds)
TilemapTests
    ✔ testBasicMapAttributes (0.043 seconds)
    ✔ testLayerQueryObjectProxies (0.042 seconds)
    ✔ testMapQueryObjectProxies (0.046 seconds)
    ✔ testQueryTilesOfType (0.038 seconds)
TilesetTests
    ✔ testContainsMethods (0.038 seconds)
    ✔ testGlobalIDQuery (0.043 seconds)
    ✔ testTilesetExists (0.046 seconds)
    ✔ testTilesetProperties (0.050 seconds)
Tests Passed: 0 failed, 31 total (1.484 seconds)

Integration:

Xcode:
- File -> Swift Package -> Add package dependency

Another Swift Package:
- Add line to `Package.swift`
  - `.package(url: "https://github.com/jdanek4/SKTiled", .branch("spm-support"))`

[mfessenden]

This looks great! Thanks for adding the tests as well.

I'll merge this, though I've been putting off support for SPM because the DebugDrawOptions optionset is defined in an Objective-C header, which SPM doesn't (yet) support.