chore(deps): bump tsdown in the npm-dependencies group (#726)

Bumps the npm-dependencies group with 1 update: [tsdown](https://github.com/rolldown/tsdown).


Updates `tsdown` from 0.20.3 to 0.21.0
- [Release notes](https://github.com/rolldown/tsdown/releases)
- [Commits](rolldown/tsdown@v0.20.3...v0.21.0)

---
updated-dependencies:
- dependency-name: tsdown
  dependency-version: 0.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
  dependency-group: npm-dependencies
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Masaaki Hirano <lambda.groove@gmail.com>

Author: dependabot[bot]

package.json

@@ -35,7 +35,7 @@
     "@tsconfig/node22": "^22.0.5",
     "@types/node": "^25.3.0",
     "knip": "5.85.0",
-    "tsdown": "^0.20.3",
+    "tsdown": "^0.21.0",
     "turbo": "latest",
     "typescript": "^5.9.3"
   },