Add "main" to package.json to fix eslint issues

[vimalloc]

Description 📖

Using eslint, I'm getting an import/no-unresolved: Unable to resolve path to module 'vite-plugin-ruby' when it clearly exists.

Background 📜

Looking at this ticket I believe that is happening because there is no main key in the package.json. Note that this error is not happening with the vite-rails-plugin, which does specify the main key.

The Fix 🔨

This updates the package.json to include the main key.

[ElMassimo]

Thanks Lily!