Add type declarations for `nodenext` module resolution

I use prosemirror-utils package in project with typescript@5. When I set compiler options `module:nodenext` and `moduleResolution:nodenext` in `tsconfig.json`, build fails:

```
error TS1479: The current file is a CommonJS module whose imports will produce 'require' calls; however, the referenced file is an ECMAScript module and cannot be imported with 'require'. Consider writing a dynamic 'import("prosemirror-utils")' call instead.
```

My guess is that typescript finds `*.cjs` scripts, expects `*.d.cts` and does not find them.
So, probably solution is to add `*.d.cts` for type declaration for commonjs environment

Start with tsconfig.json and the package's existing *.cjs scripts and declaration files to trace how TypeScript 5 resolves the package under nodenext. Reproduce the reported build failure, then verify that CommonJS consumers resolve the added *.d.cts declarations and that the nodenext build succeeds.

Written by the indexing model from the issue text.