fix: fix main entry (library build doesn't work)

Author: woothu

babel.config.js

@@ -1,5 +1,5 @@
 module.exports = {
   presets: [
-    '@vue/app'
+    "@vue/app"
   ]
 }

package.json

@@ -3,6 +3,7 @@
   "description": "Vue component wrapper for chart.js",
   "version": "0.1.0",
   "license": "MIT",
+  "main": "src/index.js",
   "scripts": {
     "build": "vue-cli-service build --target lib --name coreui-vue-chartjs ./src/index.js",
     "lint": "vue-cli-service lint",