ali teaches connor a lesson in publishing

[cnrstvns]

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[alii]

This PR

• implements required fields in package.json exports field to tell TypeScript how to resolve the types.
• uses tsup for bundling instead of tsc

Before, the types were incorrect for importing as an ESM package from an ESM runtime, or importing as a CJS package from an ESM runtime. You can run pnpm pack on an older commit, and upload to https://arethetypeswrong.github.io/ to see for yourselves.

[alii]

Fwiw you can remove the copy-types build step as tsup >= 8.0.0 (at least) correctly generates the .d.cts file. See egoist/tsup#934