Parse asmdef `versionDefines` to determine which constants should be set

[Lordfirespeed]

This should enable building patchers targeting unity 6

refs:

• https://docs.unity3d.com/6000.3/Documentation/Manual/class-AssemblyDefinitionImporter.html#version-defines
• https://docs.unity3d.com/6000.3/Documentation/Manual/assembly-definition-file-format.html
• https://docs.unity3d.com/6000.4/Documentation/Manual/upm-semver.html