[✨](headless): add side data-attributes to popover

[maiieul]

So that we could add animations when it's floating depending on where it is relative to the trigger.

For example (look at the classes far at the right):

const Panel = component$<PropsOf<typeof HeadlessPopover.Panel>>(({ ...props }) => {
  return (
    <HeadlessPopover.Panel
      {...props}
      class={cn(
        'w-72 rounded-md border bg-popover p-4 text-popover-foreground opacity-0 shadow-md outline-none  data-[open]:animate-in data-[closed]:animate-out data-[closed]:fade-out-0 data-[open]:fade-in-0 data-[closed]:zoom-out-95 data-[open]:zoom-in-95 data-[side=bottom]:slide-in-from-top-2 data-[side=left]:slide-in-from-right-2 data-[side=right]:slide-in-from-left-2 data-[side=top]:slide-in-from-bottom-2',
        props.class,
      )}
    >
      <Slot />
    </HeadlessPopover.Panel>
  );
});