feat: sanity check to validate entrypoints#669

I managed to refactor my 'out' to 'dist', not noticing 'dist' was in my .vscodeignore. All that testing, but my poblished package had no entrypoints for a few hours.

Lame mistake - yeah. But with 2 potential entrypoints and the refactoring people will be doing to support the web context, I think it's possible others may miss one of their entrypoints. Regardless - it's a very easy sanity check and I've provided a potential PR.