Use prettier to enforce a consistent style for future contributors

[rosswintle]

This is a copy of #15 but one that I have write access to and can update before merging.

[rosswintle]

@GenieTim Do you know enough about Git Hooks to make this run on pre-commit?

I can probably figure it out, but if you have the knowledge and can do it quickly that would be really cool. Don't worry if not though. Really appreciate what you have done here. 🙌

[GenieTim]

I do have some experience, but mostly by using further tools, enabling the hooks to be installed automatically e.g. when installing other dependencies. Prettier has a whole page on possibilities (https://prettier.io/docs/en/precommit.html), my suggestion would be their second one.

I exemplarily set it up in #20