Add support for fragments

[klintmane]

We should investigate wether adding support for fragments is worth it or not.

According to pomber/didact#22, the current commitDeletion:

... The trick is: if fiber.dom is null it means it's the fiber of a function component, and function components never have more than one child (Didact doesn't have Fragments like React).

Adding support for multiple children would:

It's more complex because you need to search the different dom nodes that may be at different levels on the subtrees.
See here: https://gist.github.com/pomber/64fb7e63119bef201dd8166b0fce73c4#file-didact-fiber-js-L30 (from https://engineering.hexacta.com/didact-fiber-incremental-reconciliation-b2fe028dcaec)

Edit: Found a project implementing Fragments on Didact at https://github.com/davidbarone/didact#fragments

[klintmane]

Creating a Fragment component of value null seems to do the trick.

export const Fragment = null;