ensure the node module is build when installing from github directly

when not using npmjs.org (e.g. in the case of testing a fork)

Author: hermanbanken

package.json

@@ -10,6 +10,7 @@
   "scripts": {
     "build": "tsc",
     "build:watch": "tsc --build --watch",
+    "prepare": "npm run build",
     "prepublishOnly": "rm -rf dist && npm run build",
     "postpublish": "git push && git push --tags",
     "test": "(cd tests && npm test)"