Drop CommonJS support

[ericcornelissen]

Inspired by https://antfu.me/posts/move-on-to-esm-only

Summary

Drop support for CommonJS by removing the transpiration from ESM to CJS, removing the .d.cts files, removing CJS tests, etc.

The motivation for this is to simplify the project, supported by the arguments laid out in https://antfu.me/posts/move-on-to-esm-only.

In any case, this is a breaking change for v2 of this package, and hence cannot be realized until the release of v3. This is because CommonJS support is currently supported and the package declares support for Node.js releases that don't support the new CJS-imports-ESM functionality.