[Discussion] Reducing Bundle Size

Currently the target package is `umd` when built with webpack this means that when we subsequently use this library it is non-trivial for our bundlers to optimise or tree-shake away any unused parts of the library. There are some quick wins such as making third-party imports more specific. e.g.

```javascript
// Changing this...
import { AutoSizer, List } from 'react-virtualized';
// To this...
import { AutoSizer } from 'react-virtualized/dist/es/AutoSizer';
import { List } from 'react-virtualized/dist/es/List';
```
In this example this would help bundlers (like webpack) tree-shake the `react-virtualized` library and keep the our app bundle sizes smaller.

Another really awesome thing to do and potentially more benificial would be to look adding `esm` (es modules) as another build target and have `side-effects: false` in the `package.json`. With this in place we could look at potentially hoisting out the optional internals such as `NodeRendererDefault`, `PlaceholderRendererDefault` and `SortableTreeWithoutDndContext`

Initially it could all be done in way that presents no breaking changes. What do you think?