Upgrade to Sdk-style project + Nuget package refs

Project format

Replaced traditional .NET Framework project format with the newer SDK-style, used by the modern .NET stack.

Reason for the change:

• Include all *.cs files by default. No more VS replacing include glob with the individual file includes.
• Simple XML structure. SDK projects are designed for manual editing, with good documentation provided by Microsoft.
• Support outside of the Visual Studio. VS Code has a great MSBuild project tools plugin. Makes cross-platform development easier.

RimWorld references

Replaced the previous system that tried to find and reference the game installation with the nuget package. No more VS replacing this with the hard coded paths. With this setup, RJW can be compiled without the game installed, e.g. on CI. This also makes it easier to compile for the older versions of the game, if that is worth anything.

Other mod references

Added Multiplayer and Prepatcher packages to automatically update API dlls.