Export Typescript types

At the moment, typescript types are not available to outside applications using Geppetto.

It would be nice for example to import the `Widget` type with

```typescript
import { Widget } from "@metacell/geppetto-meta-client/common/layout/model";
```

Start by locating the Widget type associated with common/layout/model and the package entry points that control what outside applications can import. Verify the intended import path from the issue, then run the project’s existing checks for package exports. Done means external applications can import Widget from @metacell/geppetto-meta-client/common/layout/model.

Written by the indexing model from the issue text.