fix: Make impossible to accidently copy JsonExporter singleton

[tony-p]

Make it impossible to copy the JsonExporter accidentally. The following snippet currently compiles and runs fine, however will not actually add the converters as it is working with a copy, so not produce the desired output.

auto json_exporter = JsonExporter::get();
json_exporter.addConverter<MyType>();

With this change it will throw a compiler error.

[facontidavide]

thanks!