Enable Nullable reference types

Since starting to use LCM with Fieldworks lite I'm running into places where NRT would help avoid runtime bugs.

Read about NRT here. Migration guide.

We need to plan how to roll this out, one huge PR is going to make merging difficult so it should be done in stages. Maybe we do it per project or maybe at a more fine grained level.

Generated code will probably be the biggest area that will be tricky to figure out.

I'm not sure how this works with dotnet framework, some things I read said it should work but it requires testing.

Finally we should consider if we want to make null issues errors or warnings. For Lexbox we found that by making them errors we actually paid attention and we largely ignored it when it was just a warning.