Windows support

[Gonsolo]

I tried out the Windows support in Swift 5.4 and got the following error:

https://github.com/kelvin13/png.git @ 3.0.2: error: invalidManifestFormat("C:\Users\Gon Solo\AppData\Local\Temp\TemporaryFile.VDuarB.swift:19:12: error: unsupported or untested platform (please open an issue at https://github.com/kelvin13/png/issues)\r\r\n #error("unsupported or untested platform (please open an issue at https://github.com/kelvin13/png/issues)\")\r\r\n ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~", diagnosticFile: nil) in https://github.com/kelvin13/png.git