Add support for JSX Fragments

Currently the shortest syntax is to add multiple JSX elements in one `append` call is to use commas:

```jsx
ui.contentView.append(
,

);
```

Very ugly. The alternative is to use WidgetCollection:

```jsx
ui.contentView.append(




);
```

However, this is much longer. In React there is also the fragment element, which would make it look like this:

```jsx
ui.contentView.append(
<>



);
```

TypeScript added support for this syntax in 2.7, but it does not work in Tabris.js because we use the `jsxFactory` compiler option. This is a limitation that may be eliminated in the near future: https://github.com/Microsoft/TypeScript/issues/20469

Once fragments work with `jsxFactory` we should support it.