Things I want to solve/merge in 3.0.

To solve these I'll be rewriting the source generation process. I will be starting out in an experimental branch
and seek feedback before bringing it into main (which is targeting 3.0 preview).

The task list is sorted from most difficult to least difficult (assumed).

• Improve performance for large number of messages (Improve codegen for large projects #48 Refactor source generation template, improve performance and update benchmarks #176)
• Improvements for large solutions (Better support for large solutions #97)
• Support generic messages (Error in generated code with generic parameter #76)
• Fix unfortunate naming conflict between namespace and concrete Mediator type (CS0118: 'Mediator' is a namespace but is used like a type #65)
• Support .NET Standard 2.0 (Support for netstandard2.0 #53)
• Improve diagnostics (Missing Request Type Exception #49)
• Reorder cancellation token parameters for functions, to realign with MediatR (CancellationToken parameters must come last #52)
• Remove uneccessary dep on System.Threading.Tasks.Extensions (System.Threading.Tasks.Extensions could be removed? #86)
• Generate code with nullable reference types enabled