(ruby/irb#886)

This is a feature that has been requested for a long time. It is now
possible to define custom commands in IRB.

Example usage:

```ruby
require "irb/command"

class HelloCommand < IRB::Command::Base
  description "Prints hello world"
  category "My commands"
  help_message "It doesn't do more than printing hello world."

  def execute
    puts "Hello world"
  end
end

IRB::Command.register(:hello, HelloCommand)
```

ruby/irb@888643467c

* master: (29 commits)
  Not all `nm`s support the `--help` option
  Emit a performance warning when redefining specially optimized methods
  YJIT: A64: Avoid intermediate register in `opt_and` and friends (ruby#10509)
  [ruby/reline] Remove not implemented method (ruby/reline#680)
  [ruby/reline] Fix vi_to_column which was broken (ruby/reline#679)
  Include more debug information in test_uplus_minus
  [Universal parser] DeVALUE of p->debug_lines and ast->body.script_lines
  Add more assertions in `test_uplus_minus`
  `super{}` doesn't use block
  fix incorrect warning.
  Update default gems list at fc36394 [ci skip]
  [ruby/optparse] bump up to 0.5.0
  show warning for unused block
  Emit `warn` event for duplicated hash keys on ripper
  [ruby/reline] Refactored Default Key Bindings (ruby/reline#678)
  [ruby/reline] Refactor waiting_proc and waiting_operator_proc (ruby/reline#649)
  [pty] Fix missing `or`
  [pty] Fix `ptsname_r` fallback
  [ruby/irb] Allow defining custom commands in IRB (ruby/irb#886)
  [ruby/reline] Support `menu-complete-backward` command for upward navigation (ruby/reline#677)
  ...