Added license icon

Author: TheLonelyAdventurer

README.md

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 A sass module loader plugin for esbuild.
 
 [![npm package version](https://img.shields.io/npm/v/@squirrelnetwork/esbuild-sass-modules-plugin?label=%40squirrelnetwork%2Fesbuild-sass-modules-plugin&logo=npm)](https://www.npmjs.com/package/@squirrelnetwork/esbuild-sass-modules-plugin)
+![license](https://img.shields.io/npm/l/@squirrelnetwork/esbuild-sass-modules-plugin)
 
 This plugin allows .scss and .sass files to be imported as modules in javascript
 files.