graphql-dotnet: GraphTypeRegistry does not contain an option for `char`

Like the title says - it’s currently not possible to use chars directly because the GraphTypeRegistry doesn’t contain a definition for char. See constructor: https://github.com/graphql-dotnet/graphql-dotnet/blob/master/src/GraphQL/GraphTypeRegistry.cs

There also isn’t a GraphType for char, so should a StringGraphType be used instead?