chore: add npm-diff.app as user of API (#1084)

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.


![external-services](https://github.com/user-attachments/assets/b94b2b64-2b15-4bb3-80e2-cc3d4d4c9a8c)

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

---------

Co-authored-by: Steven <[email protected]>

Author: oBusk

API.md

@@ -16,6 +16,7 @@ Current websites using this API:
 - [picocolors-size-benchmark](https://github.com/alexeyraspopov/picocolors/pull/76)
 - [dependencies-dashboard](https://github.com/shakib1729/dependencies-dashboard-main)
 - [https://jsgrids.statico.io](https://github.com/statico/jsgrids/commit/bb8069627d4d9a9b158ffb6324054399ef8d0433)
+- [npm-diff.app](https://github.com/oBusk/npm-diff.app/commit/d91f188458ac6eeae1cdc54f8a8e2be7fd81d7aa)
 
 ## Endpoints