add prepare script

Author: calleli

package.json

@@ -22,6 +22,7 @@
     "test": "npm run pre-test && ava --timeout=300s --verbose",
     "stresstest": "seq 1 10 | xargs -I{} npm test",
     "prepublishOnly": "npm test",
+    "prepare": "npm run build",
     "pre-test": "npm run clean && npm run format-check && npm run build:server",
     "watch": "tsc -w",
     "watch:test": "ava -w"