Add support for references from C# to XML

This PR will probably be open for a while, here's the list of features that we need to implement

[x] Creating a reference from classes that have the [DefOf] attribute back to XML
[x] Creating a reference from the text in DefDatabase.GetNamed back to XML
[x] Make sure those references work with Custom Def Classes
[x] Have the XML Def Find Usages return those C# References
[x] Add ItemProvider for autocomplete in CSharp Contexts

Garethp / Rider-RimworldDevelopment