Serhii A. Hrytsenko

Refactored `IMapper.From()` to return new interface `ITypeAdapterBuilder` instead of concrete class. FYI: Mapster.EF6 & Mapster.EFCore uses outdated package version and causes compilation errors in Mapster.EFCore.Tests and Sample.AspNetCore Closes #376

Add support for nullable reference types

1

In order to minimize `NullReferenceException`s thrown from code and to make it explicit what part of API accepts/produces `null`s I suggest adding nullable annotations. See https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references

Closes #147