Restore browser field for Vite compatibility

The browser field was accidentally removed, causing Vite to use the
raw CommonJS entry point instead of the pre-bundled UMD file. This
resulted in "Class extends value undefined" errors because Vite
doesn't polyfill Node.js built-ins like the events module.

Fixes #216

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: browningjp

package.json

@@ -18,6 +18,7 @@
     "url": "https://github.com/oozcitak/xmlbuilder2/issues"
   },
   "main": "./lib/index",
+  "browser": "./lib/xmlbuilder2.min.js",
   "type": "commonjs",
   "engines": {
     "node": ">=20.0"