fix: Rename index.js to index.mjs

This change is required to address the below error:

> This file is being treated as an ES module because it has a '.js' file
> extension and '/mnt/c/code/personal/discord-commits/dist/package.json'
> contains "type": "module". To treat it as a CommonJS script, rename it
> to use the '.cjs' file extension

action.yml

@@ -20,4 +20,4 @@ inputs:
 
 runs:
   using: node16
-  main: dist/index.js
+  main: dist/index.mjs

package.json

@@ -2,10 +2,10 @@
   "name": "discord-commits",
   "version": "1.0.0",
   "description": "A github action that nicely displays multiple commits in discord based on templates.",
-  "main": "dist/index.js",
+  "main": "dist/index.mjs",
   "type": "module",
   "scripts": {
-    "build": "ncc build discord-commits/index.js --license licenses.txt",
+    "build": "ncc build discord-commits/index.mjs --license licenses.txt",
     "test": "echo \"Error: no test specified\" && exit 1"
   },
   "repository": {