Migrate monorepo to bundle with tsdown

[delucis]

This PR migrates the monorepo to bundle using tsdown instead of crackle. This is a prerequisite for #238 in order to be able to move some packages (unpack) to ESM-only build output.

Some notes:

• Crackle provided a “dev” mode. This PR replaces that with a TypeScript custom condition as described in this article. Packages include a @capsizecss/src condition in their exports which points directly to the .ts source files. This way packages can import types/code from each other directly within the monorepo without needing to run builds. I added config to the site (Vite) and the storybook config (webpack) to handle this.

• tsdown always outputs a .d.cts for CJS type declarations, so this is a change from the .d.ts shipped currently. I don’t think this should matter at all given they are mapped in the exports and types fields in package.json anyway.

• This PR matches the existing build 1:1, shipping dual CJS and ESM code for each package. In Replace fontkit with fontkitten #238 we could then disable CJS builds for the unpack package (or drop it from other packages too eventually — especially for metrics shipping a dual bundle is pretty heavy).

[changeset-bot]

🦋 Changeset detected

Latest commit: c764b99

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@capsizecss/vanilla-extract	Patch
@capsizecss/metrics	Patch
@capsizecss/unpack	Patch
@capsizecss/core	Patch

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[michaeltaranto]

Thanks so much for driving this @delucis 🙌