chore: bump tsdown 0.21

[jycouet]

Main change is: tsdown.config.ts (and bump of tsdown)

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Latest commit: e62e4d0

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commit: e62e4d0

[manuel3108]

I still don't agree the changes that were introduced by tsdown. But since this seems to be the way forward we will probably just have to follow that for now.

Please merge if you don't see any other chances in getting rid of that config.