Add npm commands

package.json

···
   "description": "MiniZinc grammer for tree-sitter",
   "main": "index.js",
   "scripts": {
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "build": "tree-sitter generate && node-gyp build",
+    "test": "tree-sitter test"
   },
   "repository": {
     "type": "git",