Support `<slot>`

[aalin]

https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_templates_and_slots

It would be more convenient to do:

%div
  %Foo
    %Bar(slot="bar")

instead of

%div
  - bar =
    %Bar(slot="bar")
  %Foo{bar:}

... which is the current API...