Add configurable link separators with responsive visibility controls for Navigation including page list items

[sarthaknagoshe2002]

What?

Closes #75201

Adds support for configurable link separators in the Navigation block, along with options to control their visibility across devices.

Why?

Designers often need separators between navigation links for better readability and visual structure, especially in horizontal menus and footers. Currently, this requires custom CSS, which is not user-friendly or consistent across themes. This PR introduces a built-in solution that works out of the box and adapts to responsive layouts.

How?

• Introduced new block attributes for separator type and visibility settings
• Extended block wrapper classes and inline styles to pass separator configuration
• Implemented logic to convert spacing presets into CSS variables for accurate separator alignment when the block spacing is changed.
• Used CSS pseudo-elements to render separators and position them precisely between items using the computed gap
• Ensured compatibility with both Navigation items and Page List items

Testing Instructions

1. Open a post or page
2. Insert a Navigation block
3. Add multiple Navigation Link items or use a Page List block inside it
4. Open the block settings in the sidebar
5. Select a separator style (e.g. dot, pipe, dash, etc.)
6. Change the “Show on” option (Desktop only / Desktop + Tablet / All)
7. Adjust the block spacing from the Dimensions panel
8. Verify that:
   1. Separators appear between items
   2. Separators stay centered regardless of spacing changes
   3. Visibility settings correctly hide/show separators on different screen sizes
9. Preview the page and verify the same behavior on the frontend

Screenshots or screencast

Navigation.Separator.mov

Use of AI Tools

Used AI assistance to help refine code and for documentation.

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Co-authored-by: sarthaknagoshe2002 <sarthaknagoshe2002@git.wordpress.org>
Co-authored-by: henriqueiamarino <iamarinoh@git.wordpress.org>

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