chore: implement code formatting

[mcmxcdev]

Description

I saw that there was no consistent code formatting applied to the project, so I implemented that. I tried to stick to the existing code style as close as possible, but feel free to if you e.g. decide you want to have single quotes rather than double quotes everywhere.

Since gnome extensions don't have a package.json and node_modules, devs will need to have prettier installed as IDE extension to have formatting work.

Changes

• add .prettierrc.js config file
• auto-format .js, .json and .md files

[esenliyim]

This is actually something I've been wanting to implement for some time but have been too lazy to do. Thank you very much for it. Though I will be making some changes.