docs: add NPM Total Downloads Badge

Author: edelveart

README.md

@@ -6,6 +6,7 @@
   <img src="https://img.shields.io/npm/v/fignumbers" alt="NPM Version">
   <img src="https://img.shields.io/npm/types/fignumbers" alt="NPM Type Definitions">
   <img src="https://img.shields.io/npm/l/fignumbers" alt="NPM License">
+  <img src="https://img.shields.io/npm/d18m/fignumbers" alt="NPM Downloads">
 </p>
 
 [![Fignumbers Image](https://github.com/edelveart/fignumbers/blob/main/fignumber-banner.png)](https://www.npmjs.com/package/fignumbers)