Support ImplicitUsings in project and item templates

[DavidBoike]

Endpoints will use <ImplicitUsings>enable</ImplicitUsings> on target frameworks where the setting is supported. NServiceBus supports implicitly using the NServiceBus namespace. Code files generated as part of the endpoint will not emit default implicit using namespaces when this is active.

Item templates for handlers and sagas will also bind to the MSBuild value to determine whether to generate using statements. This only works if a dotnet restore has been done on the project, which Visual Studio does automatically, and which will be done as a postAction on the endpoint template, but there's not a lot that can be done about that for externally-built projects used outside Visual Studio.

[bording]

Given that I think we should also be including <Nullable>enable</Nullable>, I'm wondering if we should instead have a IsNetFramework property that could be used to handle Nullable and ImplicitUsings.

[DavidBoike]

@bording changed it for the endpoint template, where we are in control of the framework and how that turns into ImplicitUsings. But for the item templates it won't be that simple given you could be adding a handler to a net7.0 project where you've disabled ImplicitUsings.