`Encoding::Converter#insert_output` not implemented

[nirvdrum]

While working on improving the IRB test suite to run with TruffleRuby with @st0012, we discovered that Encoding::Converter#insert_output isn't implemented. I think this was overlooked because the method spec is empty, meaning we don't have any tags for it.

[eregon]

I'm a bit curious how IRB uses that method. I wish there would be no Encoding::Converter, only String#encode, but maybe that's not enough?

[eregon]

Found it: https://github.com/ruby/irb/blob/fd49135607e25eb3532f3464e7c59345b4001881/lib/irb.rb#L614-L634 Could that be done with String#encode's various options? Maybe with a :fallback Proc? If it's possible, that seems a lot simpler.