Add linter/formatting rules

[Charlie85270]

To avoid formatting differences between developers and to make the code easier to read (especially for PRs) it is advisable to use a linter configured directly in the project, for example Prettier.
Concerning the rules, each developer has preferences, personally I always use the AirBnb rules (the most known): https://gist.github.com/bradtraversy/aab26d1e8983d9f8d79be1a9ca894ab4