Create a reference CobaltCore.dll assembly for use in Github actions

[Fayti1703]

See branch fayti1703/reference-assembly.

Current status:

• We have a .csproj to create a reference assembly from a CobaltCore install. (Though this is slightly hacky, I'd love to not pull in the entire C# build config and instead just the stuff for PackageReference and running MSBuild tasks)
• We have a pre-built reference assembly in Git LFS storage. GitHub has install instructions for LFS -- might be worth adding those to the README (along with instructions on how to rebuild the assembly, which is just 'run this target in CobaltCoreReference').
• The reference assembly is currently not used in building Nickel.

[Shockah]

The reference assembly is currently not used in building Nickel.

I think that's a good thing, actually. It could be potentially used as a fallback, but being able to jump to the concrete definitions of CobaltCore members is too useful to just switch altogether.

[Shockah]

I guess it actually has to be the fallback for it to work with Github actions.

[Fayti1703]

Yup -- at minimum in CI we'd need to use the stored assembly rather than a live-generated one. Though that can also be done by setting an environment variable and doing MSBuild-conditionals based on that.