[5.x] Add custom icon selection to CP Nav Preferences

[jesseleite]

Closes #9667.
Closes statamic/ideas#995

[what-the-diff]

PR Summary

• Change icons to light versions
  All item icons are updated to use light icons for better visibility and aesthetics.
• Customization option in ItemEditor component
  Added an option to customize item icons within the ItemEditor component, but only for non-child items.
• Remove 'icon' for child items
  The 'icon' configuration is removed for new child item creation/update, allowing the use of the original icon instead of the default 'entries' icon.
• Modify NavItem icon() and new svg() methods
  The icon() method in NavItem now returns the SVG file name instead of the rendered SVG. A new svg() method is added to return the rendered version of the icon.
• Logic added to transformItems()
  Additional logic in transformItems() ensures child nav items don't have icons based on design system guidelines, while maintaining the ability to set custom inline SVGs for child items without affecting other functionality.

[tobimori]

would love an update to this!

[jackmcdade]

❤️