Add a JSON addition for Enumerator::ArithmeticSequence

[havenwood]

This PR adds a JSON addition for Enumerator::ArithmeticSequence since it was added to the Ruby stdlib.

##
# Existing Range support
require 'json/add/range'
(0..42).to_json

##
# Proposed Enumerator::ArithmeticSequence addition
require 'json/add/arithmetic_sequence'
((0..42) % 3).to_json

I forgot JSON was a gem yesterday and made a PR to Ruby and filed feature request. I realized my mistake this morning, and opened this PR. 🙂

[byroot]

Thanks for the PR but I don't wish to add any more json/add classes.

[hsbt]

FYI: https://bugs.ruby-lang.org/issues/19528