Exclude custom instance variables from State#to_h

[ydah]

Remove the leftover support for arbitrary instance variables in State#to_h after State#[] was removed in 3.0.0.

require "json"

state = JSON::State.new
state.instance_variable_set(:@custom, 42)
state.to_h.key?(:custom)
# Before (CRuby): NoMethodError
# After: false

Return only known configuration options on CRuby, JRuby, and TruffleRuby. Add regression coverage for both to_h and to_hash.

[byroot]

That's something I removed in 3.0. It's true I forgot some dead code though, but I'd rather remove the leftovers than to re-introduce this mis-feature.

[ydah]

Thanks for clarifying. I've removed the leftover instance variable handling and aligned TruffleRuby so that to_h only returns known configuration options. What do you think?

[byroot]

Thank you!