chore: add license to addons (#3238) [ci skip]

* fix(cli-ui-addon-webpack): add license info

* fix(cli-ui-addon-widgets): add license info

Author: rytswd

packages/@vue/cli-ui-addon-webpack/package.json

@@ -11,6 +11,7 @@
     "lint": "vue-cli-service lint",
     "prepublishOnly": "yarn run lint --no-fix && yarn run build"
   },
+  "license": "MIT",
   "devDependencies": {
     "@vue/cli-plugin-babel": "^3.2.2",
     "@vue/cli-plugin-eslint": "^3.2.2",

packages/@vue/cli-ui-addon-widgets/package.json

@@ -11,6 +11,7 @@
     "lint": "vue-cli-service lint",
     "prepublishOnly": "yarn run lint --no-fix && yarn run build"
   },
+  "license": "MIT",
   "devDependencies": {
     "@vue/cli-plugin-babel": "^3.2.2",
     "@vue/cli-plugin-eslint": "^3.2.2",