build(deps-dev): bump tsdown from 0.21.4 to 0.21.6

Bumps [tsdown](https://github.com/rolldown/tsdown) from 0.21.4 to 0.21.6.
- [Release notes](https://github.com/rolldown/tsdown/releases)
- [Commits](rolldown/tsdown@v0.21.4...v0.21.6)

---
updated-dependencies:
- dependency-name: tsdown
  dependency-version: 0.21.6
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]