[rubygems/rubygems] Add irb to a Gemfile for a newly created gem

I think we need this to silence the following warning when running
`bin/console` with Ruby 3.4

```
./bin/console:10: warning: irb was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add irb to your Gemfile or gemspec to silence this warning.
```

ruby/rubygems@c46230c856

Author: y-yagi

lib/bundler/templates/newgem/Gemfile.tt

@@ -5,6 +5,7 @@ source "https://rubygems.org"
 # Specify your gem's dependencies in <%= config[:name] %>.gemspec
 gemspec
 
+gem "irb"
 gem "rake", "~> 13.0"
 <%- if config[:ext] -%>