Add prettier config file

[luboskmetko]

Closes #351

[jakub300]

I have mixed feelings about adding prettier config without formatting entire project and enforcing it via eslint but it's fine.

But does it actually work, it is not valid JSON due to trailing comma?

[luboskmetko]

I wanted to add this as a quick fix because when I was working on a JS file in generator some time ago, Prettier reformatted whole file with double quotes while we use single quotes everywhere.