Forward icon class to sidebar group icon

[ghabriel25]

The scenario

Currently when using expandable sidebar group with a heading, there is no way to modify icon class (e.g. color using text-orange-500) with icon:class attribute forwarding like flux::button and flux::toggle already has.

The only workaround is to use icon slot but this might cause some issue because whenever sidebar collapsed into icon only, it uses some default class

in-data-flux-menu:text-zinc-400 
in-data-flux-menu:dark:text-white/80 
in-data-flux-menu:[[data-flux-sidebar-group-dropdown]>button:hover_&]:text-current

The problem

Sidebar group icon class was hardcoded in their component and there is no pre-setup to get it from icon:class attribute.

The solution

Defined icon class which can contains icon:class attribute value and use fixed size following flux::sidebar.item:

@php
$iconClasses = Flux::classes('size-4') // following sidebar.item which uses fixed size
    ->add($attributes->pluck('icon:class'));
@endphp

Now user can define icon:class attribute to modify the icon

<flux:sidebar.group expandable 
            heading="{{ __('My Posts') }}"
            icon="square-3-stack-3d"
            icon:class="text-orange-500 dark:text-orange-400">
    ....
</flux:sidebar.group>

Dark mode	Light mode

Note

We could support sidebar.item as well but for now I'll keep it focus on the current issue

Fixes #2843