Convert the build system to KSPBuildTools

[Phantomical]

The state of the art in KSP mod build systems has change quite a bit since the last time a commit was made to RT. This brings it up to par.

Changes:

• KSPBuildTools can now take care of the details of finding and referencing the KSP assemblies.
• Modern sdk-style projects mean we don't need to list all the source files
• We don't need any of what's in buildscripts, that can all be handled by github actions
• I am genuinely unsure if travis CI is even running anymore, and don't know now to maintain it anyway, switched to GHA
• KSPBuildTools can generate assembly attributes so we can drop AssemblyInfo.cs