check prettier formatting in validate script

[alexkrolick]

Without this, formatting is not checked in CI and only applied if the precommit hooks are installed, which is optional

Options:

• Add prettier --list-different to validate command
• Integrate prettier rules into ESLint (probably preferred for JS)

[kentcdodds]

Hmmm... I think it works already to do:

kcd-scripts format --list-different