fix: add repository field to package.json for provenance verification

The repository field is required when using npm --provenance flag.
npm verifies that the repository URL in package.json matches the
provenance statement generated by GitHub Actions.

Author: nicolaiarocci

package.json

@@ -4,6 +4,10 @@
   "description": "JavaScript SDK for the Invoicetronic API",
   "license": "MIT",
   "main": "dist/index.js",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/invoicetronic/javascript-sdk.git"
+  },
   "scripts": {
     "build": "babel src -d dist",
     "prepare": "npm run build",