Add support for Windows

[jakepetroules]

Disable tests relying on the executable since those aren't yet supported by SwiftPM, and tweak a couple platform-conditional pieces of code to support the Windows path.

[czechboy0]

Could you also enable Windows CI like on the runtime repo?

[czechboy0]

C:\swift-openapi-generator\Tests\OpenAPIGeneratorReferenceTests\FileBasedReferenceTests.swift:102: error: FileBasedReferenceTests.testPetstore : failed - 

Directory contents not equal:

  File under test: Types.swift

  Reference file: ReferenceSources/Petstore/Types.swift

  Diff output: (see below)

--- begin diff output --------------------------------------

failed: Error Domain=NSCocoaErrorDomain Code=260 "The file doesn’t exist."

--- end diff output ----------------------------------------

Seems the integration test isn't passing yet - could you take a look, @jakepetroules?