Export style in package.json

This way it can be included more easily with javascript bundlers like
browserify/sheetify

package.json

@@ -2,6 +2,7 @@
   "name": "codemirror",
   "version": "5.26.1",
   "main": "lib/codemirror.js",
+  "style": "lib/codemirror.css",
   "description": "Full-featured in-browser code editor",
   "license": "MIT",
   "directories": {