Include source files in lib/ in packed package

The source maps in `lib.cjs/` and `lib.esm/` reference the original TypeScript source files in `lib/`, so we should include them in the packed version. Then bundlers and debuggers can properly resole the source maps.

package.json

@@ -71,7 +71,7 @@
       }
     }
   },
-  "files": ["lib.cjs/", "lib.esm/", "dist/", "node", "browser"],
+  "files": ["lib/", "lib.cjs/", "lib.esm/", "dist/", "node/", "browser/"],
   "engines": {
     "node": ">=18"
   },