Remove MultiJson dependency and replace with JSON

[kojix2]

Hi.
MultiJson has ended support for Ruby versions earlier than 3.0.
Previous reports indicate that IRuby is often used with older versions of Ruby.
We will migrate from MultiJson to the standard library JSON.

[Copilot]

Pull Request Overview

This PR removes the MultiJson dependency and replaces it with Ruby's standard library JSON module to maintain compatibility with older Ruby versions, as MultiJson now requires Ruby 3.0+.

Key changes:

• Remove MultiJson dependency from gemspec and require statements
• Replace MultiJson.dump/load calls with JSON.generate/parse equivalents
• Update all JSON serialization/deserialization throughout the codebase

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
iruby.gemspec	Removes multi_json dependency from gem specification
lib/iruby.rb	Replaces multi_json require with json require
test/helper.rb	Removes multi_json require from test helper
lib/iruby/kernel.rb	Updates config file parsing to use JSON.parse
lib/iruby/session/mixin.rb	Replaces MultiJson.dump/load with JSON.generate/parse for message serialization
lib/iruby/input/form.rb	Updates form input parsing to use JSON.parse