Add scripts for when publishing @openzeppelin/contracts directly

(cherry picked from commit a1408a3411)

scripts/prepare-contracts-package.sh

@@ -5,7 +5,9 @@ cd "$(git rev-parse --show-toplevel)"
 
 # avoids re-compilation during publishing of both packages
 if [[ ! -v ALREADY_COMPILED ]]; then
+  npm run prepublish
   npm run prepare
+  npm run prepack
 fi
 
 cp README.md contracts/