[plugin_generator] move development dependencies from *.gemspec.erb to Gemfile.erb

[rogerluan]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I see several green ci/circleci builds in the "All checks have passed" section of my PR (connect CircleCI to GitHub if not)
• I've updated the documentation if necessary.
• I've added or updated relevant unit tests.

Motivation and Context

This is a follow up PR similar to #21725

This is the recommended best practice as per RuboCop https://docs.rubocop.org/rubocop/cops_gemspec.html and there was an exception disabling this rule previously.

Description

The only thing I couldn't enable was ensuring that the Gemfile.erb gems are sorted alphabetically, because apparently even if I add - Include statement for Gemfile.erb in the rubocop config file, it will ignore it and will only look for Gemfile, .gemspec and .gems files in custom directories 😞 The gems are already sorted alphabetically, though, and since this file pretty much never receives updates, this is good enough.

I also added a brief comment about what each dependency does. I think this is great for newcomers and helps lower the contribution barrier 😊 which is especially helpful when users are developing new plugins.

Testing Steps

To test this branch, modify your Gemfile as:

gem 'fastlane', git: 'https://github.com/fastlane/fastlane.git', branch: 'rogerluan/move-dev-dependencies-to-gemfile-in-plugin-generator'

And run bundle install to apply the changes.

Then run:

bundle exec fastlane new_plugin this-is-my-test

Screenshot demo'ing this working (notice the Gemfile was generated correctly, including the version placeholders):

[lacostej]

🚀

[Garciakj4]

I no