Add prettier to the project

[squgeim]

Prettier formatting is controversial but I believe is one of the most "agreeable" formattings out there, and that the whole of Facebook's source has been moved to it is a strong point.

Besides, formatting will always be a case of taste and that will defer from person to person. This usually leads to conflicts between people in a code review (comments like "put a space after if" and another "don't put a space after if").

Arguing about formatting in a code review takes the focus away from the actual code, and to how the code looks to one person.

Adding prettier as a linting rules takes the formatting variable out the code review. Everyone, whether they like it or not, will follow the same rule.