Pillbox: support indicator="checkbox" like flux:select does

[kobykorman]

flux:select variant="listbox" multiple accepts indicator="checkbox", so a multiple select shows empty boxes on every row. You can tell it takes several values before the first pick. flux:pillbox is always multiple, but it has no indicator setting: its rows show the checkmark only, which appears only once a row is picked. Until then, a pillbox's pane looks the same as a single select's.

The pillbox is already half-wired for this. pillbox/options.blade.php declares an indicator prop, but nothing reads it: pillbox/option.blade.php renders a fixed <flux:pillbox.indicator />, whose only look is the check.

Suggested change (mirrors what select.option already does):

1. flux:pillbox accepts indicator and makes it available to its options, the way flux:select does.
2. pillbox/option.blade.php:

   @aware([ 'indicator' ])
   ...
   <div class="w-6 shrink-0 [ui-selected_&]:hidden">
       <flux:select.indicator :variant="$indicator" />
   </div>

   (or give pillbox.indicator the same check / checkbox / radio variants as select.indicator)
3. Add indicator to the option's unsafe list, as Fix Blaze safe list in select option #2626 did for select.option, so a bound value (:indicator="...") isn't folded away.

It already works when placed by hand. Putting <flux:select.indicator variant="checkbox" /> inside a pillbox option's slot renders correctly, since the indicator keys off ui-option[data-selected]. That takes one CSS override to hide the built-in check column, which is what we're shipping as a stopgap until this lands.

Flux 2.21.0 (livewire/flux-pro 2.21.0), Livewire 4.4.8.