(https://github.com/ruby/irb/pull/788)
Similar to Pry, it displays user-defined aliases in the help message with
a dedicated section. With the current default aliases, it looks like:
```
...other sections...
Aliases
$ Alias for `show_source`
@ Alias for `whereami`
```
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environment
(https://github.com/ruby/irb/pull/647)
This can:
- Make it easier to scroll up and down the commands list
- Avoid pushing up users' previous output
- Allow users to do basic search with `/<word>`
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