[List Block] - Add support for block gap

[Mayank-Tripathi32]

Resolves #69308

What?

This PR implements the blockGap support for the core/list block, allowing control over spacing between list items.

Why?

The core/list block currently lacks blockGap and layout support, making it difficult to control the spacing between core/list-item blocks. Since core/list-item is a separate block, the gap should be adjustable through blockGap, ensuring better design flexibility and consistency with other blocks that support spacing control.

How?

• Added supports: { spacing: { blockGap: true } } to the core/list block.
• Added Layout support and disabled editing.
• Ensured that the blockGap value is applied correctly in the block's styling.

Testing Instructions

1. Open the Block Editor (Gutenberg).
2. Insert a List block (core/list).
3. Add multiple List Item (core/list-item) blocks inside it.
4. Adjust the blockGap value using the spacing controls in the sidebar.
5. Verify that the spacing between list items updates correctly both in the editor and frontend.
6. Save and refresh to ensure the setting persists.

Screencast

Screen.Recording.2025-02-26.at.8.11.53.PM.mov

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