Schema files as ES modules

**Is your feature request related to a problem? Please describe.**
Currently schema files seem to have slightly different rules for globals and package names (e.g. `cube()` and `import Funnel from 'Funnel'`).

**Describe the solution you'd like**
It would be awesome if I could import modules using Node's existing rules, so a schema might look like this instead:

```javascript
import { cube, Funnels } from '@cubejs-backend/schema-compiler';

cube({
extends: Funnels.eventFunnel
...
});
```

Hopefully by using the ES6 module-loading mechanism, CubeJS might get to do less work with respect to setting up/parsing of schemas.

**Describe alternatives you've considered**
Not sure there are any, other than to stick with what exists.

**Additional context**
This might also make it easier for TypeScript support in schema files, since there would be no need to parse imports.