Remove dependency on Newtonsoft.Json from Config.Json

[natemcmaster]

cref dotnet/announcements#90

Let's remove our dependency on Newtonsoft.Json from the Json configuration provider and use System.Text.Json. This will unblock dotnet/aspnetcore#4260.

[natemcmaster]

cc @ajcvickers @HaoK. This is something we should do for 3.0. The sooner the better so we can provide corefx with feedback on the new Json APIs.

[ajcvickers]

@natemcmaster @DamianEdwards @Eilon

@HaoK and I discussed this yesterday and we don't believe it is possible to implement under the current plan. Specifically, since Config is in Extensions, and Extensions must still target .netstandard2.0, and the new JSON APIs will not be available on .netstandard2.0, there's no way to use them.

[Eilon]

Should we get in a room to discuss?

[natemcmaster]

Yes, I think we should.

[ajcvickers]

@Eilon Yep. But I think the discussion is wider than just this. There are other packages where we are saying we want to both support new features and .NET Framework. /cc @divega

[Eilon]

Yup agreed.

[HaoK]

Due to some differences in the new json stack and Json.Net for back compat, we are going to add another package that continues to use Json.NET see: #1075

The main difference between the two is loss of support for single quotes:

            var json = @"{
                'setting': {
                    'hello': 'a',
                    'bob': 'b',
                    '42': 'c',
                    '4':'d',
                    '10': 'e',
                    '1text': 'f',
                }

[HaoK]

Added via #1028