deps: upgrade dev dependencies

[willybrauner]

Migrate the build from tsup to tsdown and update all dependencies to their latest version.

tsup is no longer maintained and can't emit declaration files with TypeScript 7, so the build was broken after the dependencies update.

• Build: tsup is replaced by tsdown (rolldown). The published files and the public API are the same: interpol.js, interpol.cjs, interpol.d.ts, interpol.d.cts and their sourcemaps.
• Minification: still done by terser (as a rolldown plugin) to keep properties mangling, the bundle size is unchanged (~3.5kB brotli).
• Dependencies: TypeScript 7, Vite 8, Vitest 5, jsdom 30, size-limit 14, changesets 3, pnpm 12.
• Tests: jsdom 30 drops invalid CSS declarations like a browser does, styles() tests now use valid unitless values (0) to check autoUnits = false. The styles() behavior is unchanged.
• Remove codesandbox ci, no longer compatible

[changeset-bot]

🦋 Changeset detected

Latest commit: 1084759

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

This PR includes changesets to release 1 package

Name	Type
@wbe/interpol	Patch

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[willybrauner]

size-limit report 📦

Path	Size
@wbe/interpol	3.44 KB (-0.17% 🔽)