chore: add npm-diff.app as user of API

My little app for comparing npm releases uses Packagephobia to include extra information on the comparison page to show if the installsize has changed between the two releases. Performs packagephobia for both packages, shows difference between them.  I've been bad with User-Agent until today, when I changed it to this; https://github.com/oBusk/npm-diff.app/blob/d91f188458ac6eeae1cdc54f8a8e2be7fd81d7aa/src/lib/api/user-agent.ts