[Fix #10731] Show tip for suggested extensions that are installed but not loaded in .rubocop.yml

[nobuyo]

Fixes #10731.

This PR adds a new tip for suggesting extensions that are installed but not loaded.

# Gemfile

# frozen_string_literal: true

source('https://rubygems.org')

git_source(:github) do |repo_name|
  "https://github.com/#{repo_name}"
end

gem 'minitest'
gem 'rake'
gem 'rspec'
gem 'rubocop'
gem 'rubocop-rake'
gem 'rubocop-rspec'

# .rubocop.yml

require:
  - rubocop-rake

AllCops:
  NewCops: enable
  TargetRubyVersion: 3.1

Layout/LineLength:
  Max: 40

Run rubocop:

$ bundle exec rubocop
Inspecting 2 files
..

2 files inspected, no offenses detected

Tip: Based on detected gems, the following RuboCop extension libraries might be helpful:
  * rubocop-minitest (https://rubygems.org/gems/rubocop-minitest)

The following RuboCop extension libraries are installed but not loaded in config:
  * rubocop-rspec

You can opt out of this message by adding the following to your config (see https://docs.rubocop.org/rubocop/extensions.html#extension-suggestions for more options):
  AllCops:
    SuggestExtensions: false

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[nobuyo]

@bbatsov @koic I'd be happy if you take a look at this :)

[bbatsov]

Sorry for the slow response - I like it! Thanks for working on this! 🙇‍♂️