files such as images don't work with ESM style imports

At the moment to import images in Dojo you have to use require.

For example say we want to use an image in a widget, we must do it like so:

```typescript
const bottomImage = require('./../images/placeholder.png')
```
It would be consistent and idomatic to be able to do image imports as we do with modules like this:

```typescript
import bottomImage from './../images/placeholder.png';
```