Add css file to exports map

[edoardocavazza]

Hello!

Currently, the CSS file cannot be imported if using the node resolution algorithm for module packages because the alert.css is not exported in the package.json.

Package subpath 'undefined' is not defined by "exports" in /Users/edo/Projects/chialab/catalog/node_modules/remark-github-blockquote-alert/package.json.

I would suggest to change the "exports" field to

"exports": {
  ".": "./lib/index.js",
  "./alert.css": "./alert.css"
},

[jaywcjlove]

@edoardocavazza Upgrade v1.2.1