IRB v2.0

[st0012]

This is the meta issue for collecting breaking changes we're going to make in IRB v2.0

• RubyLex: Remove deprecated toplevel ::RubyLex #1155
• Multi-irb commands: Deprecate multi-irb feature #653
• LOAD_MODULES feature
• USE_RELINE and USE_REIDLINE en vars
• IRB::Context#prompt_n[=] methods
• ExtendCommandBundle