LottieGen MSBuild Task Issues

The MSBuild task for LottieGen has a couple of issues:

1. The NuGet package only adds the Task, it provides no infrastructure for using it to generate the code
2. The task assumes the input path is a relative path, simply appending the output path in front of the full file passed in
3. The task also assumes the input path does not have an extension, just blindly appending to the file name for the output files list
4. The output files parameter has the incorrect file names