Add Prettier

[coliff]

Prerequisites

• I have searched for duplicate or closed issues
• I have read the contributing guidelines

Proposal

It can be a bit annoying when the formatting of code changes a bit with PRs. I think it'd be good to add Prettier as a dev dependency and add the following script to package.json

"prettier": "prettier --write ./*.{js,json,md,yml}"

We should ignore the dist folder though.

Motivation and context

Keeps docs and code formatted consistently and prettier.

[roblarsen]

That's fine with me.