add support for vitest / jest

[jantimon]

find a way how next-yak does not prevent users from writing ui components tests

[jantimon]

Possible solution

We could split the code so that all APIs which get transpiled by the Babel plugin (like styled, keyframes, css, etc.) to a new next-yak/internal module.
Here's how it would work:

1. Our Babel plugin would automatically replace imports like import { styled } from "next-yak" with import { styled } from "next-yak/internal"
2. We'd keep a test-friendly version of these APIs in the main next-yak export

Benefits

For our users:

• Vitest/Jest would "just work" out of the box
• No need for extra configuration in their test setup
• Potentially better / optimized snapshot tests without hashes

For us maintainers:

• Cleaner typings: Currently, our type definitions are a bit messy. The import { styled } from "next-yak" statement has different meanings before and after compilation. By separating into two different exports, we can properly type both scenarios:
  • Pre-compilation: The external API that users interact with
  • Post-compilation: The internal API that represents how the code actually runs
• Improved unit testing:
  • We could structure our tests with separate input.tsx (before compilation) and output.tsx (after compilation) files
  • input.tsx would import from "next-yak" (simulating user code)
  • output.tsx would import from "next-yak/internal" (representing compiled code)
  • This setup would give us proper type checking for our compilation results in output.tsx
• With a clear separation, we can better document what's intended for public use vs. what's for internal operations, potentially reducing confusion for contributors

Potential Downsides

• Bundle size reporting on sites like Bundlephobia might not be accurate anymore
• Users who don't set up the compilation steps correctly might see the styles "working" (no errors) but not actually being applied

Side Nodes

• next-yak/internal must not publish its types to dist (so users won't get autocomplete to next-yak/internal`