Update tsdown to latest version

[teamchong]

Summary

Updates tsdown to 0.22.0.

Also:

• Uses tsdown --config-loader tsx so older Node 22 builds can still load tsdown.config.ts.
• Replaces deprecated external with deps.neverBundle.
• Marks bun external because Bun declarations reference Bun types.
• Updates the config type from Options to UserConfig.

Validation

• mise x node@22.12.0 -- npm run build
• npm run build
• npm run test:types
• npm run test:bun
• Node dist smoke test
• Bun dist smoke test
• Local Worker smoke test with wrangler dev

Note on config loader

tsdown@0.22.0 requires ^22.18.0 || >=24.0.0. Older Node 22 versions, for example 22.10, fail with the default config loader before tsdown.config.ts is loaded (Failed to import module "unrun").
The build scripts use tsdown --config-loader tsx so this update does not raise the repo’s build-time Node requirement yet.

Closes #173

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Latest commit: 832b92a

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[teamchong]

I have read the CLA Document and I hereby sign the CLA

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commit: 832b92a

[dimitropoulos]

please just document (by adding to the PR description) the specific finding on 22.10 and needing the tsdown flag in the package.json so people in the future can find out why it's there.