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

[ctheune]

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):

<span data-assignees="a b c"
  _="init
          set $assignees to my @data-assignees split by ' '
          if not $assignees then set $assignees to []" />

I wish I could do this instead:

<span data-assignees="a b c"
  _="init
          set $assignees to []
          extend $assignees with my @data-assignees split by ' '
        " />

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.