Add solution folders to organise VS project.

[mr-russ]

This enables easier grouping of projects and readability when working on different areas.

Folders are;
Backends
Libraries
Tests

[kenkendk]

I think .Net Core (or at least VS Code) has switched a bit back towards using .sln files, so this is a good change.
Unfortunately, the .sln files have a tendency to cause merge conflicts, and this PR got caught in the massive .NET8 merge.
@mr-russ if you can resolve the conflicts (or simply re-do the work) then I would be in favor of merging.

And happy PR anniversary!