Add support for border to navigation link

[dhananjaykuber]

What?

Closes #61746 – Nav Block: Add Support for Separator Block & Borders

This PR adds support for border and padding controls to the core/navigation-link block.

Why?

Currently, there's no way to visually separate navigation links within the navigation block using borders. This enhancement allowing users to style navigation links with borders and spacing to better match custom designs.

How?

• Updated the block.json of core/navigation-link to include support for:
  • __experimentalBorder with color, style, and width
  • spacing.padding

Testing Instructions

1. Go to the Site Editor or create a new post/page.
2. Insert a Navigation block.
3. Add a few Navigation Link items inside it.
4. Select a Navigation Link and check the sidebar (Block settings).
5. Confirm you can now apply:
   a. Border color, width, and style
   b. Padding around each individual link

Screenshots or screencast

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Co-authored-by: dhananjaykuber <dhananjaykuber@git.wordpress.org>
Co-authored-by: bhubbard <bhubbard@git.wordpress.org>

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[t-hamano]

Thanks for the PR!

Let me close this PR as a duplicate of #66483,