Add "extend" to help putting multiple items from one list into another

In Python we have "list.extend(otherlist)" as a multi-item "append" basic function.

I found that this is a bit tedious in hyperscript (or of course I might be missing something). Especially if I'm parsing something where the result could be a non-list (which is also interesting in its own from a typing perspective that splitting an empty string results in a non-list):

```

```

I wish I could do this instead:

```

```

We could also consider whether "split by" should always return a list (I'm not quite sure what's going on there when `@data-assignees` is an empty value) but that's orthogonal to the extend ability.