Add prepare script to src/ for git-based installation

When installing chain-sdk from a git URL with &path:src, package managers
extract only the subdirectory. This prepare script goes up to the repo root
and runs the TypeScript build so dist/ exists.

Author: douglance

src/package.json

@@ -47,6 +47,9 @@
   "repository": "git+https://github.com/OffchainLabs/arbitrum-chain-sdk.git",
   "author": "Offchain Labs, Inc.",
   "license": "Apache-2.0",
+  "scripts": {
+    "prepare": "cd .. && npm run build"
+  },
   "engines": {
     "node": ">=18"
   },