npm build has no module exports, so it can't be imported with a bundler

Hello,

We're looking to switch to matgui, but we stumbled upon some integration issues.
An ES-module build (via module/exports in package.json) would be great for embedding in vite/webpack/esbuild, so we could do:

```typescript
import Yasgui from '@matdata/yasgui'
```

Greetings,
Frank