Support for Central Package Management

[tintoy]

It looks like CPM functionality is now being surfaced via the .NET upgrade assistant so it may be worth a bit of investigation to see how much effort it will be to support.

See https://devblogs.microsoft.com/dotnet/dotnet-upgrade-assistant-cpm-upgrade/ for details.

More information about CPM:

https://learn.microsoft.com/en-us/nuget/consume-packages/Central-Package-Management

Language-server side of this work tracked by tintoy/msbuild-project-tools-server#93.

Supersedes #103.