Added prepublishOnly hook (#8)

Co-authored-by: Sergey Andreev <sergey.and@okwork.io>

Author: elenik72

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@cadolabs/observable",
-  "version": "3.0.0",
+  "version": "3.0.1",
   "description": "Observable library",
   "repository": "https://github.com/Cado-Labs/observable",
   "author": "Aleksei Bespalov <nulldefiner@gmail.com>",
@@ -14,7 +14,8 @@
     "build": "rollup -c",
     "lint": "eslint .",
     "test": "jest",
-    "test:coverage": "jest --coverage"
+    "test:coverage": "jest --coverage",
+    "prepublishOnly": "yarn build"
   },
   "devDependencies": {
     "@babel/cli": "^7.17.10",