Update dependency tsdown to ^0.21.0

Author: renovate[bot]

package.json

@@ -252,7 +252,7 @@
     "semantic-release": "^21.1.2",
     "stylelint": "^16.26.1",
     "svgo": "^4.0.0",
-    "tsdown": "^0.18.2",
+    "tsdown": "^0.21.0",
     "tsx": "^4.21.0",
     "typescript": "^5.9.3",
     "unist-util-is": "^6.0.1",