Use ESLint to improve code style consistency

[HabuTheTiger]

To improve the code quality and keep a consistent style I'd like to start using ESLint. While a config seems to exist, it's not installed and no GitHub action exists to check if changes to the code still preserve the style.

Tasks:

• Add ESLint to the javascript project
• Update the config to the new standards (since ESLint 9)
• Fix any errors that ESLint finds
• Add GitHub action for ESLint