[Feature]: Add accessible Carousel Counter block

[theMasudRana]

Title: Add accessible Carousel Counter block

Description

Add a new Carousel Counter block for displaying the active carousel position in a compact current/total format, for example 1/6.

The counter should integrate with the existing Embla Carousel v8 setup and WordPress Interactivity API state so it updates as the carousel selection changes.

Requirements

• Add a new child block: rt-carousel/carousel-counter
• Display the counter as current/total, for example 1/6
• Use Embla v8 APIs for count state:
  • selectedScrollSnap() for the current index
  • scrollSnapList() for the total snap count
• Include the counter in the default carousel navigation group
• Add frontend styles using BEM classes:
  • .rt-carousel-counter
  • .rt-carousel-counter__current
  • .rt-carousel-counter__separator
  • .rt-carousel-counter__total
• Provide CSS variables for customization
• Ensure the block is accessible:
  • Visible text remains compact, e.g. 1/6
  • Screen readers get a descriptive label, e.g. Slide 1 of 6
  • Avoid duplicate live announcements because the carousel already has a live region
• Register the block in PHP
• Add/update tests for frontend counter callbacks and block registration
• Ensure compiled build assets are generated

Acceptance Criteria

• A new Carousel Counter block is available inside carousel blocks
• New carousels include the counter by default
• Counter updates correctly when navigating slides
• Total count reflects Embla scroll snaps, not just raw slide nodes
• Screen readers can identify the counter as Slide X of Y
• npm run build passes
• Relevant JS/PHP tests pass
• CSS lint passes