Nested Groups don't work with typescript

Hi to all!

I want to include "vis-timeline" in my typescript project

When I try use "nestedGroups" the component "Timeline" break when I write this.timelineEl = new Timeline(appDiv, items, groups, {});

Meantime it work if I write this.timelineEl = new Timeline(appDiv, items.get(), groups.get(), {});

Make sure to mention which vis-component (network, timeline, graph2D, graph3d) you are referring to.
```
"vis-data": "^6.3.0",
"vis-timeline": "^7.3.7",
"vis-util": "^3.0.0 || ^4.0.0"
```
Make sure to mention which browser and OS you are using when creating a bug report.

chrome/windows10

Please provide a minimal code example that demonstrates your issue. We recommend using jsbin for that.

Please look at raws 89 and 92

thanks in advance

Stefano

visjs / vis-timeline