Include artifacts in npm package (#103)

* Include artifacts in npm package
* Add a `prepublishOnly` script that ensures that the contract artifacts are updated before publishing the package

package.json

@@ -6,7 +6,8 @@
   "license": "AGPL-3.0-or-later",
   "files": [
     "/contracts/**/*.sol",
-    "!/contracts/z_mocks"
+    "!/contracts/z_mocks",
+    "/artifacts/*.json"
   ],
   "scripts": {
     "migrate": "truffle migrate --network development --reset",
@@ -24,7 +25,8 @@
     "compile": "yarn buidler compile --force",
     "generate-typings": "rimraf ./types/generated && typechain --target 'truffle-v5' --outDir types/generated './build/contracts/*.json'",
     "prettify": "prettier --write test/*.ts test/**/*.ts types/generated/*.ts types/*.ts artifacts/*.ts",
-    "flatten": "sol-merger \"./contracts/**/*.sol\" ./_flat"
+    "flatten": "sol-merger \"./contracts/**/*.sol\" ./_flat",
+    "prepublishOnly": "yarn run test-prep"
   },
   "repository": {
     "type": "git",