Feature request: support multiple count targets in SelectionFormController

[sobakasu]

Motivation:
to be able to use the 'count' of selected items in button text or other customised selection text.

specifically this code:

  countValueChanged(count) {
    this.element.toggleAttribute("hidden", count === 0);
    this.countTarget.textContent = count;
    this.singularTarget.toggleAttribute("hidden", count !== 1);
    this.pluralTarget.toggleAttribute("hidden", count === 1);
  }

was thinking something like this:

  countValueChanged(count) {
    this.element.toggleAttribute("hidden", count === 0);
    this.countTargets.forEach((el) => el.textContent = count; });
    this.singularTargets.forEach((el) => el.toggleAttribute("hidden", count !== 1); });
    this.pluralTargets.forEach((el) => el.toggleAttribute("hidden", count === 1); });
  }

The other problem is how to use this feature nicely in the view.
(not nice example)

  <%= table_selection_with(collection:) do %>
    <%= tag.button formaction: "#", formmethod: :get do %>
      <span data-<%= Katalyst::Tables::Selectable::FormComponent.new(collection:).send(:form_target, "count") %>>0</span>
      Hello
    <% end %>
    <%= tag.button "Activate", formaction: "#", formmethod: :put %>
  <% end %>