Block API: add blockGap support when skipping spacing serialization

[DarkMatter-999]

What?

Closes #63163

This PR adds support for blockGap when using supports.spacing.__experimentalSkipSerialisation in the block.json.

Why?

Currently there exists support for margin and padding but blockGap is not supported. This PR adds support for that.

How?

This PR adds gap.ts at packages/style-engine/src/styles/spacing/gap.ts.
The implementation is similar to the implementation for the padding and margin.

Testing Instructions

Create/edit a block.

1. In block.json, set supports.spacing.__experimentalSkipSerialisation to true.

"supports": {
	"spacing": {
		"__experimentalSkipSerialization": true,
		"padding": true,
                 "margin": true,
		"blockGap": true
	}
}

2. In Edit import { __experimentalGetSpacingClassesAndStyles as useSpacingProps } from '@wordpress/block-editor';.
3. In Edit const spacingProps = useSpacingProps( attributes );

import {
	__experimentalGetSpacingClassesAndStyles as useSpacingProps,
} from '@wordpress/block-editor';

export default function ( { attributes } ) {
	const spacingProps = useSpacingProps( attributes );
	console.log( spacingProps );
}

Screenshots or screencast

Before:

Screen.Recording.2025-06-05.at.11.11.42.AM.mov

After:

Screen.Recording.2025-06-05.at.11.12.41.AM.mov

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