Port configuration source generators to incremental

[chyyran]

https://github.com/dotnet/roslyn/blob/main/docs/features/incremental-generators.md
https://andrewlock.net/creating-a-source-generator-part-1-creating-an-incremental-source-generator/

If we can implement this at a property-level scope, we should endeavour to extract an API out so that orchestration plugins can use them and generate config.

[chyyran]

A big drawback + advantage is that configuration templates have to be compiled. This is similar to how Windows handles metadata for projections but makes it difficult to handle things like Cemu graphics packs that are updated via XML.

We need to provide primitives to generate a configuration template AST from aribtrary parsed ASt.