(https://github.com/ruby/irb/pull/934)

Since commands can't be chained with methods, their return values are
not intended to be used. But if IRB keeps storing command return values
as the last value, and print them, users may rely on such implicit
behaviour.

So to avoid such confusion, this commit suppresses command's
return values. It also updates some commands that currently rely on
this implicit behaviour.

https://github.com/ruby/irb/commit/fa96bea76f

(https://github.com/ruby/irb/pull/824)

* Command is not a method

* Fix command test

* Implement non-method command name completion

* Add test for ExtendCommandBundle.def_extend_command

* Add helper method install test

* Remove spaces in command input parse

* Remove command arg unquote in help command

* Simplify Statement and handle execution in IRB::Irb

* Tweak require, const name

* Always install CommandBundle module to main object

* Remove considering local variable in command or expression check

* Remove unused method, tweak

* Remove outdated comment for help command arg

Co-authored-by: Stan Lo <stan001212@gmail.com>

---------

https://github.com/ruby/irb/commit/8fb776e379

Co-authored-by: Stan Lo <stan001212@gmail.com>

(https://github.com/ruby/irb/pull/881)

https://github.com/ruby/irb/commit/83d90550c2

(https://github.com/ruby/irb/pull/873)

* Replace ExtendCommand with Command and standardize command related names

1. Rename lib/irb/extend-command.rb to lib/irb/command.rb
2. Rename lib/irb/cmd/*.rb to lib/irb/command/*.rb
3. Rename test/irb/test_cmd.rb to test/irb/test_command.rb
4. Rename ExtendCommand to Command

* Alias ExtendCommand to Command and deprecate it

* Rename Command::Nop to Command::Base

* Not deprecate old constants just yet

* Add lib/irb/cmd/nop.rb back

https://github.com/ruby/irb/commit/462c1284af