(https://github.com/ruby/irb/pull/877)

* Make help command display help for individual commands

Usage: `help [command]`

If the command is not specified, it will display a list of all available commands.

If the command is specified, it will display the banner OR description of the command.

If the command is not found, it will display a message saying that the command is not found.

* Rename test/irb/cmd to test/irb/command

* Add banner to edit and ls commands

* Promote help command in the help message

1. Make `show_cmds` an alias of `help` so it's not displayed in the help message
2. Update description of the help command to reflect `help <command>` syntax

* Rename banner to help_message

https://github.com/ruby/irb/commit/43a2c99f3f