Add prettier

[mdubus]

What's wrong ?

Prettier missing. Many developper use the "format on save" option in their IDE, and without a code formatter and a configuration file, it can be pretty time-consuming trying not to break the current format.

What's inside this PR ?

Addition of prettier as dev dependency + configuration file, and formatting of every file regarding the same specification. Please take a look at the rules to see if it seems coherent to you 🙏

[bidoubiwa]

bors merge

[bors[bot]]

Build succeeded:

• linter-check
• tests