Add common namespaces to global using

Once we move to .NET 6 and C# 10 we should add some common namespaces to a file with global usings in our project templates. Alternatively add them to the .csproj file.

Suggest that we start with the following namespaces:

EPiServer
EPiServer.Core
EPiServer.DataAbstraction
EPiServer.DataAnnotations

episerver / content-templates

Add common namespaces to global using #38