Prepare code for development

[javierav]

This PR prepares the code for the planned developments:

• Add rubocop with standard gem to ensure code consistency and style, also running it in the CI.
• Add a Gemfile for development dependencies, a best practice.
• Add the development gems' binstubs to the bin folder to avoid having to prefix commands with bundle exec.
• Adds support for direnv, a utility that allows, among other things, to automatically add the bin folder to the PATH environment variable every time we enter the project directory, closely related to binstubs. This allows us to directly use the rake command in the console and actually execute bin/rake, which in turn is like writing bundle exec rake.
• Adds an IRB console in bin/console with the preloaded library for testing in development.
• Set the gem to require ruby ​​>= 3.0
• Fix all rubocop warnings.
• Cleanup gitignore file.

[adrianthedev]

This is awesome!
A lot of improvements here!

There's one thing I'd like us to be a little bit loose on.
Let's use standard as it provides a lot of good defaults and prevents us and future commiters to do a lot of yak shaving and fiddle with the linting rules.

Thanks a lot @javierav

[javierav]

@adrianthedev changes ready!