3.0 tracking issue - rewriting source generation

[martinothamar]

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 (#48)
• [ ] Improvements for large solutions (#97)
• [ ] Support generic messages (#76)
• [ ] Fix unfortunate naming conflict between namespace and concrete Mediator type (#65)
• [ ] Support .NET Standard 2.0 (#53)
• [x] Improve diagnostics (#49)
• [x] Reorder cancellation token parameters for functions, to realign with MediatR (#52)
• [x] Remove uneccessary dep on System.Threading.Tasks.Extensions (#86)
• [x] Generate code with nullable reference types enabled