adds npm prepare script

package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "watch": "watch 'npm run build' src",
     "test": "echo \"Error: no test specified\" && exit 1",
-    "build": "tsc --build ./tsconfig.json"
+    "build": "tsc --build ./tsconfig.json",
+    "prepare": "npm run build"
   },
   "repository": {
     "type": "git",