chore(deps): update dependency tsdown to ^0.21.0 (#541)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package.json

@@ -136,7 +136,7 @@
     "sass": "^1.52.2",
     "stylus": "^0.64.0",
     "svelte": "^5.0.0",
-    "tsdown": "^0.20.0",
+    "tsdown": "^0.21.0",
     "typescript": "~5.9.0",
     "typescript-eslint": "^8.5.0",
     "vite-plugin-eslint4b": "^0.7.0"