[spike] Try out tsdown

[LekoArts]

Description

After trying out https://tsdown.dev/ in a couple of OSS projects of mine I was curious what it would take to migrate our usage of tsup to it here.

In my personal testing I saw that with isolatedDeclarations you run into problems like microsoft/TypeScript#58944 where complex computed types (e.g. through Zod) would need to be written out. In general, enabling this option would effectively force us to enable https://typescript-eslint.io/rules/explicit-function-return-type/. Not necessarily a bad thing to do, but something we have to decide on.

tsdown is I think nearly on par with tsup in terms of what we need from it (except for the edge cases). I was excited to read in https://tsdown.dev/guide/dts that also declaration map generation is available. We need those for Typedoc generation.

Here's a speed comparison on my personal MacBook:

tsup

➜ hyperfine 'pnpm build && pnpm build:declarations'
Benchmark 1: pnpm build && pnpm build:declarations
  Time (mean ± σ):      2.247 s ±  0.048 s    [User: 4.370 s, System: 0.265 s]
  Range (min … max):    2.184 s …  2.348 s    10 runs

tsdown

➜ hyperfine 'pnpm build'
Benchmark 1: pnpm build
  Time (mean ± σ):     778.6 ms ±  26.9 ms    [User: 1268.8 ms, System: 160.9 ms]
  Range (min … max):   745.8 ms … 822.1 ms    10 runs

As you can see, much faster! The types package is a good candidate for the isolatedDeclarations option but even without it tsdown should generally be faster. Especially if we need to generate declarations for it, too.

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

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